Paging has nothing to do with this. When I say 1 MB thread stacks, it means the maximum size of that thread's stack in the virtual address space [0]. Each of these million threads could be using only a few KBs for its stack (out of that 1 MB of stack space). That would imply a few gigs of physical memory => no paging.

[0] http://man7.org/linux/man-pages/man3/pthread_attr_setstacksi...