It seems like the code doesn't actually contain the RF firmware, though - it's just the glue to talk to an external chip which essentially acts as a "modem".
The reason ZMK uses the MIT license - and proper RF support cannot be added to QMK - is that Nordic's RF stack drivers use a license which isn't compatible with GPL. This means if you want to do wireless stuff inside the actual keyboard firmware you can't use QMK.
There is of course a fork out there doing exactly this because nothing is technically stopping you, but you're legally not allowed to combine the two.
