Board Thread:General Discussion/@comment-30388913-20170828193135/@comment-32182236-20180416175030

"A monster cannot absorb the SOULs of other monsters. Just as a human cannot absorb a human SOUL... So then... What about something that's neither human nor monster?"-Entry 7, True Lab

Flowey is neither human nor monster. He is a flower. He absorbed the human and monster SOULs at the same time. Because he was neither, he could absorb both. He was NOT a monster.