Board Thread:Site Discussion/@comment-32623270-20170719125809/@comment-25501246-20170719163127

Ojiro's Quirk is Mutant, but it has a mechanism of control that is similar to an Emitter.

A Quirk is defined by way more than just one thing. There's a bunch of mechanics that make them work like they should, which are collectively called the "Quirk Factor". Shota can't erase mutations, but if the mutation requires a degree of control, then that control can be erased, as it is part of the Quirk Factor that Shota can interrupt with his Quirk.