Well, it's true that そうではない has a usage that you showed, but I don't think it is the main use case of that expression.
The real difference between those two is in the focus of negation. Verb + そう can be roughly translated as "(likely) about to V", where そうではない negates the whole phrase ("not [about to V]"), while そうにない the mood part only ("not likely" or "not about to'").
In actual examples:
When you are a scout watching the enemy camp, you can say both way. #1 means that you have some confidence that they are not ready to perform an attack, so that you can relax guard, at least for a good while. However, #2 means that you are still not sure whether they are ready to attack; maybe they could attack us in any time, just not immediate (in a moment).
Now, this case:
is as if saying "That person is not (even) about to leave. (S/he's already left!)". This is a quite special and rhetorical context.
Due to the specific time-bound meaning, ～そうにない is only usable when following a verb, and not adjective. See the @snailplane's answer to Confusion about “Seemingly not ～”.