[usrusr]

What if he actually is an apostrophe?

Disregarding the semantic absurdity (and the e/i mismatch), the rule works fine: "his an apostrophe" should look a lot more wrong to anyone with enough understanding of English that its/it's mistakes make a difference than "he's an apostrophe".

"He's" also only ever means "he is" or "he has", just like "it's". That's the entire point of this little memory aid.