From a very cursory look, it seems like it doesn't. Based on the man page for mmap [1], it seems like all MAP_32BIT does is ensure that memory is allocated in the lower 2GB of addresses.
Notably, this means the actual pointers to that memory are still 64 bit.
Obviously, when you change pointers to be 32 bit, that requires an ABI change because the binary layout of your data structures is different.