In chapter 4 of the book it says:
The sentence $a=a$ is necessarily true. So of course, no matter what your initial premises may be, it will be impossible for those premises to be true and for $a=a$ to be false--simply because it is impossible for $a=a$ to be false!
It then goes on to say a few pages later:
it should be clear, however, that not all logically necessary claims are tautologies. The simplest example of a logically necessary claim that is not a tautology is the FOL sentence $a =a$. Since this is an atomic sentence, its truth table would contain one T and one F. The truth table method is too coarse to recognize that the row containing the F does not represent a genuine possibility."
How can this sentence be such that "it is impossible to be false" yet not be a tautology?
edit: nvmd, I see what they're saying.