In theory, Jay is correct, but assuming that theory will always work in practice is, in this case, how linemen end up dead. We're all well aware of never assuming theory = practice, but admittedly the stakes tend to be a little lower in our world.
Ensuring that a generator physically cannot backfeed is just one layer of protection against the already very high risk of the job of a lineman. Then there is, of course, checking for the presence of voltage before starting work, but it's possible for a generator to start AFTER this check.
Another layer of protection is grounding all conductors prior to beginning work, so that if power does come back (via the grid or a backfeed) A: The lineman and bucket is not the best path to ground and B: The source is tripped.
Reading through that forum post, it sounds like that particular contractor had a reputation for lacking proper safety precautions, so one or more safety layers may have been removed, making the risk/impact of any single mistake much greater than it should be.
-Matt