The standard literally could not care less if there's a kernel underneath. It doesn't care how malloc is implemented or what the kernel, if any, looks like "from its perspective". The perspective that has relevance is that of the caller of malloc. From your own quotes:

> The pointer returned if the allocation succeeds is suitably aligned so that [...]

> If the space cannot be allocated, a null pointer is returned.

If you use mmap and "from your perspective" that's considered success then it's your perspective that's faulty. The standard is literally telling you right here^ there are 2 possibilities: either you allocate the space and return a pointer to the space, or you don't and you return null. There is no third option of "space cannot be allocated but you return non-null anyway". That's quite literally the end of the story.