Well there's a pipe dream if I ever heard one! I'd expect it to just pass Forza 4 sales at best! (currently at 4.03mill)
It's possible considering he said the last 2 Forza games which is Horizon and F5. Horizon is only at 1.53M and F5 just came out. However, even if you wanted to use the "main installments" with F4's 4.03 and F5 might still work. If F5 doesn't get it's annual bundle next christmas then it's sales won't be at 4M. It might be then closer to 3M if that. Even then with GT legs, it's quite possible.