Whats the easiest way to optimize a program for your product if you already have a compiler that does that for you?
Granted it doesn't 100% mean they used it, but it would be pretty stupid to rewrite and insert the same Intel code into the compiler thats going to be used.
As far as optimizing for one and not the other, you check the cpu id flags and NOT THE CPU VENDOR. There is no reason to check the vendor string other than to cripple the code, ie sse2 for AMD or AVX for GenuineIntel.
I love the Intel excuses that "AMD's SSE doesn't work". Its not AMD's SSE, its Intel's that AMD purchased with the cross liscence agreement, only to be disabled yet again by the Intel compiler.
Is that how business is supposed to work? Liscence a technology only to have it disabled by the same company that you paid?
Funny, I don't see AMD disabling x86-64 on GeniuneIntel, but maybe they should since its ok for Intel to do that.