There’s no current user who needs both the Makefile generator and C++20 modules support.
The Unix Makefiles generator is focused on POSIX-compatible Makefile platforms. Such platforms do not support C++20 compilers to begin with, and any platforms which have C++20 compilers have ninja trivially available.
There’s simply not a compelling use case. If there were GNUMake specific generator, I think it would be compelling to add C++20 module support to it.