Mojo is a Python superset, Python is written in C therefore mojo is written in C.
I don't know what that rust library they benchmarked against is but given that they claim to have performed 50% better I would be sceptical. Given that most sources benchmark rust, C and C++ at about the same level.
I don't think you can trust because we can't verify the results they put out, plus iirc, people have done wild comparisons between mojo and different langs.