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.