I think he's referring to C specifically, not C++. It's true that modern versions of MSVC are compliant (and they're also typically faster at implementing features than gcc and clang), but for the longest time there were subtle differences in their C library. To this day I don't think they support VLAs, which are technically standard C (At least until recently, I'm not sure about the latest versions, hopefully someone more knowledgeable can say more).