My understanding from Mojo's plans is that they want to compile all of Python via their compiler (eventually), and then extend Python with extra syntax that will compile to high performance. I think right now they might not compile all of Python yet, so you are right they are neither a subset nor a superset, but once they deliver on their plans, they will become a superset.
A particular advantage of subsetting the language is that LPython inherits all the tooling of Python. I use pudb and PyCharm daily to develop LPython code.
Both are valid and consistent approaches with their pros and cons, I listed some of them here: https://fortran-lang.discourse.group/t/fast-ai-mojo-may-be-t....