Seems Revolver IS canon

[Parzival]

There's been a lot of speculation over the years on whether or not Red Dead Revolver is canon to RDR and RDR2. Well, I just encountered a stranger at a camp (you know, one of the rare ones that don't threaten to shoot you if you get too close...) in Hennigan's Stead about a stone's throw west of the legendary bison's map doodle, we conversed and he said he used to be a bounty hunter, how it wasn't all it was cracked up to be, and then went on to talk about Red Harlow (mentioning him by name) and his quest for revenge in Revolver.

Looks like it's confirmed now.