A player said that there is an errata on jaguar revolver but it is not listed in the rulebook. He said that instead of having "X equals your momentum" text, it now has "X is equal to your momentum when this card is played". Is it true?

There's a few bans/erratas that haven't reached the tournament rules yet because it's being revamped heavily by James.

Yes, that's how Jaguar Revolver is currently ruled to work, the original wording causes too many problems and basically makes the game go completely haywire.