This is an old problem but since the source of the OP states a more general result that the stated in the OP, I think it is worthwhile to present the complete result and a proof as presented by the source:
In what follows, $X$ and $Y$ are linear topological spaces (linear spaces equipped with a topology in which addition and scalar product are continuous operations, and singletons are closed), and $f$ is a function from $x+U$ to $Y$, where $U$ is some neighborhood of $0\in X$.
Theorem: Hadamard directional differentiability implies compact directional differentiability. Conversely, if $f$ admits a compactly directional derivative $f'_x$ at $x$, and if $f_x':X\rightarrow Y$ is sequentially continuous on $X$, then $f$ is directional differentiable in the sense of Hadamard.
Proof:
The direct part (which is the one the OP is concerned) is based on the fact (see the comment section) that if $f$ has Hadamard derivative $f'_x$, the $f'_x:X\rightarrow Y$ is sequentially continuous. Assume $f$ is not compactly differentiable at $x$. This means that there exists a compact set $K\subset X$ and an open neighborhood $V$ of $0\in Y$ such that for any $n\in\mathbb{N}$, there is $(t_n,h_n)\in (0,\tfrac1n)\times K$ such that
$$
\begin{align}
\frac{f(x +t_nh_n)-f(x)}{t_n}-f'_x(h_n)\notin V\tag{0}\label{zero}
\end{align}
$$
Let $W$ be a symmetric neighborhood of $0\in Y$ such that $W+W\subset V$. By the compactness of $K$, we may assume that $h_n\xrightarrow{n\rightarrow\infty} h$ for some $h\in K$ (the fact that $X$ is $T_1$ separable linear space -hence Hausdorff- implies that every sequence in a compact subset of $X$ admits a convergent subsequence). As $f$ is Hadamard differentiable, we have that there is $n_0\in\mathbb{N}$ such that
$$
\begin{align}
\frac{f(x +t_nh_n)-f(x)}{t_n}-f'_x(h)&\in W \quad n\geq n_0\tag{1}\label{one}\\
f'_x(h_n) -f'_n(h)&\in W\quad n\geq n_0\tag{2}\label{two}
\end{align}
$$
It follows from \eqref{one} and \eqref{two} that
\begin{align}
\frac{f(x +t_nh_n)-f(x)}{t_n}-f'_x(h_n)&=\Big(\frac{f(x +t_nh_n)-f(x)}{t_n}-f'_x(h)\Big) +\\
&\quad\quad \Big(f'_x(h) -f'_n(h_n)\Big)\in W+W\subset V
\end{align}
for all $n\geq n_0$ in contradiction to \eqref{zero}.
Conversely, assume that $f$ admits a compactly directional derivative $f'_x:X\rightarrow Y$ which is sequentially continuous. Let $(t_n,h_n)\in (0,\infty)\times X$ a sequence such that $t_n\xrightarrow{n\rightarrow\infty}0$ and $h_n\xrightarrow{n\rightarrow\infty}h$. The set $H=\{h_n,h:n\in\mathbb{N}\}$ is a compact subset of $X$. For any open neighborhood $V$ of $0\in Y$ choose a symmetric neighborhood $W$ of $0\in Y$ such that $W+W\subset V$. By assumption, there are $\delta>0$ and $n_0\in\mathbb{N}$ such that for all $0<t<\delta$
$$
\begin{align}
\frac{f(x+th)-f(x)}{t}-f'_x(h)\in W,&\qquad (t,h)\in(0,\delta)\times H\tag{4}\label{four}\\
f'_x(h)-f'_x(h_n)\in W,&\qquad n\geq n_0\tag{5}\label{five}\\
t_n\in(0,\delta),&\qquad n\geq n_0\tag{6}\label{six}
\end{align}
$$
Combining \eqref{four},\eqref{five}, and \eqref{six}, we obtain that for all $n\geq n_0$
$$\begin{align}
\frac{f(x+t_nh_n)-f(x)}{t_n}-f'_x(h)&=\Big(\frac{f(x+t_nh_n)-f(x)}{t_n}-f'_x(h_n)\Big)+\\
&\quad\quad \Big(f'_x(h_n)-f'_x(h)\Big)\in W+W\subset V
\end{align}
$$
This shows that $f$ has Hadamard directional derivative $f'_x$.
Comments:
Definition 0: $f$ is directionally differentiable at $x$ in the sense of Gâteaux if there is a function $f'_x:X\rightarrow Y$ such that for any $h\in X$
\begin{align}
\lim_{t\rightarrow0+}\frac{f(x+th)-f(x)}{t}-f'_x(h)=0
\end{align}
Definition 1: $f$ is directionally differentiable at $x$ in the sense of Hadamard, if there is a function $f'_x:X\rightarrow Y$ such that for any $h\in X$ and any $h_t\xrightarrow{t\rightarrow0+}h$
\begin{align}
\lim_{t\rightarrow0+}\frac{f(x+th_t)-f(x)}{t}-f'_x(h)=0
\end{align}
(Notice that $x+th_t\in x+U$ for all sufficiency small $t$.)
Remark 1a: It is a simple exercise to show that this definition is equivalent to saying that for any sequence $(t_n,h_n)\in (0,\infty)\times X$ such that $(t_n,h_n)\xrightarrow{n\rightarrow\infty}(0,h)$,
\begin{align}
\lim_{n\rightarrow\infty}\frac{f(x+t_nh_n)-f(x)}{t_n}-f'_x(h)=0
\end{align}
Remark 1b:
When $X$ is a normed space, it is an easy exercise to show that differentiability at $x$ in the sense of Hadamard is equivalent to
\begin{align}
\lim_{(t,k)\rightarrow(0+,h)}\frac{f(x+tk)-f(x)}{t}-f'_x(h)=0
\end{align}
for all $h\in X$.
Remark 1c: Clearly differentiability in the sense of Hadamard implies differentiability in the sense of Gateaux. Furthermore, the function $f'_x$ is positive homogeneous, that is $f'_x(\alpha h)=\alpha f'_x(h)$ for all $\alpha\geq0$ and $h\in X$. In most applications, it is also required in the definitions above that $f'_x$ be a linear bounded operator from $X$ and $Y$.
Definition 2: $f$ is compactly differentiable at $x$ if there is a function $f'+x:X\rightarrow Y$ such that for any compact $K\subset X$
\begin{align}
\lim_{t\rightarrow0+}\frac{f(x+th)-f(x)}{t}-f'_x(h)=0
\end{align}
uniformly on $K$. (Notice that since $K$ is compact, it is bounded and so, $x+tK\subset x +U$ for all $t>0$ small enough.)
The main result used on the proof above is the following
Lemma: If $f$ has directional derivative $f'_x$ at $x$ in the sense of Hadamard, then $f'_x:X\rightarrow Y$ is sequentially continuous.
Proof:
Let $\{h_n:n\in\mathbb{N}\}$ be a sequence in $X$ that converges to some $h\in X$. For any open neohborhood $V$ of $0\in Y$, let $W$ be a symmetric open neighborhood of $0\in Y$ such that $W+W\subset V$. Since Differentiability in the sense of Hadamard implies differentiability in the sense of Gâteaux differentiability, we have that for any $n$ there is $0<t_n$ such that $t_n\xrightarrow{n\rightarrow\infty}0$ and
\begin{align}
\frac{f(x+t_nh_n)-f(x)}{t_n}-f'_x(h_n)\in W\tag{7}\label{seven}
\end{align}
Differentiability in the sense of Hadamard on its own implies that there is $n_0\in\mathbb{N}$ such that
\begin{align}
\frac{f(x+t_nh_n)-f(x)}{t_n}-f'_x(h)\in W,\qquad n\geq n_0,\tag{8}\label{eight}
\end{align}
Combining \eqref{seven} and \eqref{eight} we obtain for all $n\geq n_0$
\begin{align}
f'_x(h_n)-f'_x(h)&=\Big(\frac{f(x+t_nh_n)-f(x)}{t_n}-f'_x(h)\Big) -\\ &\qquad\quad\Big(\frac{f(x+t_nh_n)-f(x)}{t_n}-f'_x(h_n)\Big)\in W+W\subset V
\end{align}
This proves that $f'_x$ sequentially differentiable.
