I kind of doubt this. Polyphony is a notoriously slow developer, they took 6 years to make GT5 and it took them 4 years to make GT Sport, which had just 168 cars on release. I find it hard to believe they managed to make GT7 in 3 years while supporting GT Sport with quite a lot of DLC (the car count has doubled since release and the track count more than doubled). Only way I can see this being true is if GT7 is a cross gen game that builds on what is already in GT Sport, rather than a built from the ground up PS5 game.
The cars in GTS (and some of the graphics tech) are forward compatible with future GTs. That was the entire point of of 'rebooting' things with GT Sport. To make future GTs faster to pump out.
Whether that happens or not is another story, but GT7 being a launch or 2021 game isn't out of the realm of possibility.
But like I said in a previous comment, cross-genning GTS makes a lot of sense to me, so whatever GT7 is is going to be interesting in light of GTS and PS5 playing PS4 games