No, that does not cause a heap allocation on its own. If other lines of code in that function caused a pointer to that value to escape the lifetime of the current function's stack frame, the compiler would determine that it has to be heap allocated instead.

I believe the person you are replying to was making a confusing point about some hand wavy notion of "any kind of allocation", which includes stack allocations... which are determined at compile time, not with "alloca".