Why doesn't assimilate transform equipped non-Darkin champions into the Darkin unit of the equipped weapon or level them up?

"Assimilate" has a specific wording with "non-champions".

I do agree with you, it says "champions equipped with darkin level up instead", not sure if bug when other champs holding it doesn't level up

because it doesnt include champions

Something important to note is that keywords are often not always precise in what they do. (For example, Overwhelm says "Excess damage I deal to my blocker is dealt to the enemy Nexus". It doesn't say that only Attack strikes, and not Single Combat style strikes, count. It doesn't say that it goes through Barrier. It doesn't say that the Nexus damage is counted as part of the strike, and will proc Nexus Strike, but won't proc Strike twice)

The idea is that after seeing it in action, you should know what they do, and they don't want to scare away new players trying to learn the game with an overly complex description of a keyword (which is already something that adds complexity to the game), just to be technically correct.