Yeah, there’s all kinds of problems with that supposed “theme” people are connecting.
Ultra Burst is hardly much of a mechanic unto itself. It’s basically just a reskinned Mega Evolution except for a single Pokémon. The new battling feature in Gen 7 was very clearly Z-Moves, which doesn’t fit the “unit prefixes” mold at all.
Some people have said “Ah, but Z-Moves don’t change form; this naming theme is only for form-changing mechanics.” But if that’s the case, then what about Primal Reversion? If the lone example of Ultra Burst counts, surely Primal Reversion, which can be used by
two Pokémon, should also be included? But “Primal” obviously isn’t a prefix.
Okay okay, so scratch Ultra Burst and Primal Reversion, since they only exist for one or two specific Pokémon anyway. That gives us Mega-Giga-Tera, which is the actual progression of the prefixes, hooray! We’ve cracked it! Gen X’s battle transformation will surely be called “Peta Power” or something!
Except… the “Giga” (which I think is already a bit of a stretch because it’s not called “Gigamax”) in our supposed sequence only exists in English.

In Japan, Gigantamax is called Kyodaimax.