> while Mojo is a strict superset of Python

Is it superset if it doesn't implement all of Python? It seems more like a set that has a non-empty intersection with Python.

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.