I believe it’s bcs when they’re born (or at least in the early stages of their lives), they get mandatorily tested if they’re an esper or a guide. If they’re neither, they get left alone to mingle w society, if the test becomes positive, they get pulled out of society to train them.
So since MC was just a regular guy w a regular job, he’s someone who mutated into a guide, rather than being born as one.

Can someone help me explain why jaejin was considered a mutant rather than an awakener? I kind of skip some of the political stuff