Johann Bernoulli did not fully understand logarithms?

This wikipedia article makes the claim:

"Bernoulli's correspondence with Euler (who also knew the above equation) shows that Bernoulli did not fully understand logarithms."

This is found under "History" . What does wikipedia mean here? Surely, Bernoulli understood what a logarithm was.

• Pure speculation on my part, but perhaps they mean complex logarithms. The logarithm $\log:\mathbb{R}^+ \to \mathbb{R}$ is much simpler. – nullUser Jul 14 '13 at 3:32
• @nullUser looks like you were right – Ovi Jul 14 '13 at 3:48

According to Bradley and Sandifer, eds., Leonhard Euler: Life, Work and Legacy, pages 259-260, Bernoulli, in correspondence with Euler, 1727-29, asserted $\log(-x)=\log x$. There's more to it than that, the book goes into some detail.