What do you mean when you say it exhausts the heap? I understand that the stack size is fixed but can't the heap grow unbounded using sbrk till it exhausts the address space?

I think they mean that it grows to the point where it collides with the virtual address space allocated to another block (the stack, a shared library, a mmap'd area etc)

I don't know anything aboug tcmalloc specifically but if the program is not mapped at 0, then there's space below it as well. The heap can meet the stack using sbrk while memory below where the program is could still be mmapped. This was the case on x86-32 anyway. On x86-64 programs are mapped much lower (on my system anyway.)