So by my logic - if you tried this exploit and it failed the first time, then worked the second time: No one else has tried it before you. Otherwise it would either have worked the first time (if you guessed the same pass) or not worked at all (if the first time it was tried a different pass was used).

Or is this not a permanent password set?

Well, I suppose if someone had exploited your system with this, they could probably install some remote access tool, and then disable the root account and unset the password, and remove all evidence they were there.

But, if you don't have Screen Sharing or Remote Management enabled and exposed to the WAN, you're probably safe unless someone untrusted had physical access.

It's hard to know how long this vulnerability was "known." The initial report on Nov 13th looks second hand, so it may have been circulating earlier.

If that's true (and certainly sounds plausible from what is known so far), that's a very valuable heuristic.