Just because jemalloc has a mutex.c file, that doesn't mean that common paths aren't meant to be lock free and in the case of lots of little allocations that can go into small bucket sizes in jemalloc they should be.

It is still putting your head in the sand since at some point they have to go to the OS and map in memory which should lock and lots of small allocations are a terrible way to anything for performance, but it is possible to have some paths in an allocator not have locks.

Also if there are thread local heaps, those won't lock either.