Surface conditions is not the geometry: Oumuamua has spent thousands to millions of years nowhere near the heat of a star, in the interstellar medium. So unlike cyclic comets in orbit around Sol, having an unusual surface chemistry would be expected since it's not being evaporated every few hundred years by close encounters.

The point is, pleading about potentially naturally occurring conditions we've never seen before occurring in an object we've never seen before isn't much of an argument when you have no evidence to actually exclude them (and the observation timeline was in a phase where it was already faint).