Except that the monk's "unarmed strikes count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage."
The monk's unarmed strikes in consequences bypass the immunity, but since "resistance from magical weapons" explicitly *isn't* "overcoming resistance [...] to nonmagical attacks and damage", the Demilich's resistance doesn't apply either.
In other words by RAW the monk's strikes do the full damage.
There are many reasons to not follow RAW (chief among them being "I prefer my rule"), but this isn't one of them.