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Another cool thing to consider is that missingno allowed the appearance/creation of pokemon up to level 255 back in red and blue.
Considering the ultra beasts are actually the shit'mons of their dimension, and yet are at such a high level, may imply that level 100 is not the levelcap in their home dimension.
Being bathed in energy of the ultra wormhole could easily explain the temporary level boost (temporary in the sense that they snap back to level 100 the second they gain any real exp) up to 255 and the power that came with it.
