Vent all you like!
Hmm, you say apparently, so it's only hearsay, right? At this point, I'd file the info away and not forget about it, but take it with a pinch of salt.
If down the line you definitely get overlooked for promotion, I'd ask why at that time, but I'd not bring this bit of gossip up. I think it would only muddy the waters and create unnecessary drama.
It's hard to shrug stuff like this off, I know. But unless you heard it with your own ears all the other person would do is deny it anyway.

All you can do is keep doing your own job well, and see what the future brings. This new manager might not last very long anyway. :O