Multithreaded compression in xz on macos
looks like it was just my filesize was too small (~160mb), when using the flag -9 anyway. It was explained to me like this:

"If you use -9, it means it uses 64 MiB LZMA2 dictionary and the default block size is 3 * dict_size = 192 MiB, a new thread is started for every 192 MiB of input."

So at -9 I would need a filesize of 1344+ MB to get all 8 cores active, each working on a >192MB block of data