The "Powered by PS5" shows a build of GT7 with around 428 cars and was uploaded in mid-December (suggesting the build may have been around that time or November). Obviously, that's not even a final build just yet, and who knows what else they're working on as of the moment. After all, as of the time of writing, it's 39 days before launch, so only a matter of time before it goes gold.
The "over 400" can range anywhere from that 428-car figure, should they choose to release it that way, or perhaps something even higher, like the high 400's (around 450+ to 490+), but below 500. Whatever they want to add in the game at launch, they will add it. It's the same thing when GT Sport launched, they promised over 140 cars, then over 150, and in the end, it launched with 167 cars.
That probably isn't the final number just yet and they're still likely going to put in a few more cars before the game goes gold. Either way, that's beyond my knowledge, and I hope we get the final car list for launch soon, with some surprises in there.