Everything in C operates according to the "as if" rule. In other words, the resulting program merely needs to behave
as if the result was executed according to the spec. How execution happens is left entirely up to the compiler.
C also doesn't define "the stack" in any way. And of course alloca() isn't part of C at all, but on the systems where it exists, it's not very specific about just where it allocates stuff, just that it's on that nebulous "the stack".
Given the above, I believe you cannot write a conforming program that can detect the difference between alloca(0) returning a region of 0 bytes at the end of the stack and alloca(0) returning something else. Since no conforming program can detect it, the "as if" rule allows the return value to be considered to be anything at all.