Singing requires no theory. In fact, most singers don’t really know any theory, they just sing what sounds good. You need to play an instrument or know how to compose music to be a real musician.
I’m not calling out Hayley Williams in particular, so none of fanatic fanbase should come charging.
Your vocal chords are an instrument and one of the most difficult to master.
^
People who think this LEGITIMATELY do not understand what singing IS.
It’s not EASY. It requires training and work. So shut up.