I'd be staggered if Sony release Horizon 2 on anything other than PS5 until they no longer see it as being useful in bringing people over to PS5. I.e. once it's down to $14 on PSN store and most/all people who have a PS5 and want to will have played it and moved on.
I also find it very unlikely that anyone here is going to dig around and find out any firm facts. Any strategic plans on this games beyond its release on PS5 and the initial plans for DLC will likely be known only at the highest level (those people tend not to blab - well, not often) and any long-term plans will unlikely be set in stone anyway.
Look, if anyone is going to hold off purchasing a PS5 so that - in X number of years - they can purchase a new PC (latest Gfx cards, etc.) to play Horizon 2 clearly don't buy into the saying, "Time Flies". Seriously!
If you like the look of Horizon 2 and are interested in a PS5 then grab one, enjoy the heck out of the game and don't look back.
I personally think the setting looks great, know that the team are technically up there with the best in the business and confident that it'll justify (along with the other games we know Sony are capable of making) buying a PS5 as soon as I can get my grubby hands on one. Power of PC or Xbox Series X doesn't change that.