I think that's correct. Ferrari is also conspicuously absent. Ditto Lamborghini, Maserati and a few others you'd expect.
Forza Motorsport is vastly better (and it has quite a bit in the way of Porsche offerings). I can't really justify going out and blowing $600-$800 on a gaming console though.
GT4 is still good fun. Several of the Benz racecars are exceptional. The Audi TT race car is a blast too. It was so much fun it actually got me down to test drive one for real (I'd have bought it too if I didn't think it was overpriced).