Found out I might have unknowingly cheated after term completion
Assuming that the no-collaboration rule was really not communicated to you, then you didn't cheat.  You followed the rules as you reasonably understood them based on the information you were given.  There's no such thing as "unknowingly  cheating"; it requires intent.

However, there is a substantial risk that the professor may think you have cheated, if he mistakenly thinks he made it clear to the entire class that collaboration was not allowed.  So it is in your best interest to clear it up before any formal charges are considered.  Just explain what you've said here: you were told that the rules would be "the same" as the midterm (if you have this in writing, better yet), and you later found out that other students were told something different.

The fact that the term has ended means nothing.  It is entirely possible, in all academic systems I know of, to impose a punishment for cheating even after the term is over, perhaps months or years later.   And it is not too unusual that investigations can't be completed before grades are due, in which case the professor may assign the grade based on an assumption of no cheating, but it can be changed retroactively if his investigation confirms wrongdoing.

So even if you don't want to "dig it up", the professor still can, and you will convey more honesty if you get out ahead of it.  You have a good defense, but it won't look as good if you have to say "well, I found out afterwards there might be a problem, but I didn't say anything because I hoped you wouldn't notice".

Note that your acquaintance definitely did cheat, if he's telling the truth about what the professor told him: he'd been clearly informed that collaboration was not allowed and he did it anyway.  Moreover, he did it in a way that placed innocent students at risk of being suspected and prosecuted for cheating.  Many people would feel that you also have an obligation to report this to the professor.