In some languages you can put a second person conjugation next to a noun that might otherwise use third person verbs, and it serves as implying that you are that noun. I'm not sure if older forms of English had that construct. I think many Indo-European languages do.

The part of the lord's prayer that says "our father who art in heaven" is kinda like this - father is linked to a second person conjugation. You could remove some words and make it into "father art in heaven", which you claim is ungrammatical. I'm skeptical that it was.