Can isAlive() be False immediately after calling start() because the thread hasn't yet started?

It can't happen, at least not in CPython's implementation. That comes from staring at the code for Thread.start (here from the Python 3 source, but it doesn't matter):

def start(self):
    ...
    try:
        _start_new_thread(self._bootstrap, ())
    except Exception:
        with _active_limbo_lock:
            del _limbo[self]
        raise
    self._started.wait()

_start_new_thread() is implemented in C, starting a new thread and running self._bootstrap() inside that new thread. self._bootstrap() in turn invokes self.run(). If that's all there were to it, then the invoking thread could indeed return an arbitrary amount of time before run() started to execute. But the:

    self._started.wait()

at the end blocks on an internal Event. The bootstrap code sets the _started Event shortly before invoking run(), and the state of that same event is the primary thing isAlive() looks at.