I think what @Josh meant was that because RDR1 was never released on PC, those with only a PC are unable to continue the story from after RDR2.Wait, what? You don't get the extra bit at the end? Don't want to say to much here for fear of spoiler. That's an interesting decision.
I've also just finished RDR2 and found it very hard going initially, however by the end really wished it was not yet over. I'd rate it at 9/10, this is arguable the most immersive open world ever created, but OMG the shooting and melee mechanics are like something from an early 2000s game.