Purely sticking to facts:
Forza horizon 5 can't be "the first next gen game", not by facts and not by opinions.
why? simple, forza horizon 5 isnt a next gen game, it is cross gen game. Because it is available on xbox one too.
Basically the same if ratchet & clank would have been released on ps4 -> it would not be really a next gen game either.
Does it look like a next gen game on series x is a different deal, and it can be discussed from different viewpoints and by opinions.
IMO it looks good, but it probably looks much better IRL vs. youtube, like games usually do. So it probably looks like a next gen game, while it is a cross gen game.
At least to me, next gen literally means a game tailored to next gen (current gen now) system, without having last gen version available. And how games look, it is separated thing from gens.
When I will buy series x, I will get flight simulator + forza 5, so it is good enough to me graphically.