Isn't saying muonium is "a hydrogen isotope" sort of like saying a car is "a horse isotope"?
I thought proton equivalency was the fundamental definition of an isotope, though perhaps that's just a lie you feed chemistry undergrads.
To describe it as "a hydrogen isotope" is just about the optimal mix of uninformative and misleading!
For the purposes of explanation, the antimuon-electron pair is chemically similar enough to a proton-electron pair to call it an isotope. It has a +1 massive center, with a probability distribution for a less-massive -1 charge cloud.
Deuterium and tritium, two true isotopes of hydrogen, increase the mass of the center by adding neutrons. An antimuon is, I think, about 10% the mass of a proton. So muonium would act a lot like extra-light hydrogen for the instant before it decays.
It would have been clearer to call it an "exotic atom, chemically similar to hydrogen", though. That wouldn't conflict with the common definition of isotope as an atom with the same number of protons as the reference atom, but with a different number of neutrons.
I'm actually not aware of any exotic atoms that are not chemically similar to hydrogen or antihydrogen, as getting an exotic helium-like atom would require forming a +2 exotic center and making two -1 charges interact with it before it decays. That is likely beyond our current capabilities.