You’re thinking in terms of location, rather than state-of-being. “I’m home” is your status.
“I’m driving, I am bored, I’m safe, I am away”… None of those sound weird, do they? This, combined with the more technical grammar rules others have commented…
Home is the adjective. It’s a state of being.
Many times I’ll walk in the door but need to log into work, and I’ll say to my wife “I’m not home yet”. As in, my external responsibilities are not completed and I am not available. When I’m available to my family or to relax, I have then become “home”.
Edit: I meant adverb. It modified the state of being. Like being “away”.