He had a snack before? Ok Umbra I missed that.
the difference between a normal symbiote and grim is that he takes magic powers, things that are the S.H.A.Z.A.M. for Satoshi, while he can't steal them he copies them which degrades the TM style characters.
reason 2 isn't meta, it's a part of his character. He reads, specifically myths, hero books and records.
reason four is but as a player I have the right to keep my characters defined.
bottom line is, he only knows two black creatures, Angra (who was screwed by gravity trap) and the symbiote. He doesn't know it's Peter's directly but there is a whole planet of them after all.