I'm not downplaying FM at all; I think it looks pretty great in actuality especially for having in-game car damage and in-game realtime RT, both are pluses over GT7.
But the people saying it "destroys" GT7 visually are exaggerating massively IMHO. Environmental detail might be better on the actual tracks themselves, but trackside detail seems like a wash between the two. Same with car model details (again minus FM having the advantage of realtime RT and car damage). In terms of lighting I think GT7 is superior and by a considerable margin, even ignoring there's no realtime RT in-game. PD just seem to have a better taste for controlled, realistic lighting, and their color palettes are a big advantage in that regard as well.
Those two things, color palette and lighting, basically even things out between FM and GT7 visually, in spite of some of the advantages the former has. GT7 just seems like it looks and feels more "lived in" in terms of overall visual fidelity, meanwhile FM still has a bit too much of a cartoonish look that's part of their choice of color palette and lighting style tho that's been kind of a thing with the series since the first game.
Overall tho which one you prefer is probably down to subjective preference.