In the sense of the kernel object code in core, yes, that's true.
In the sense of affecting the behavior of the CPU, the only difference I see is that object code is external, where microcode is internal to the CPU. You can verify that the object code under the GPL doesn't run counter to your intentions, and you can also improve it or tailor it to your purposes. Why can't you do the same with microcode? It is also executable, just on a different level.
I'm not necessarily arguing for opening up Intel's microcode format (although dang, that would be sweet for my curious mind). I'm simply saying Linus is a bit two-faced for arguing against nVidia's proprietary module, but including Intel's microcode.