Alone At Home

when i go home sometimes, I am alone.
By that I mean none of my housemates are in the house and sometimes they both are inside so I am not alone.

If they are not inside they are outside.

They are outside

Therefore, they are not inside.

The above argument may be valid, but only if the claim "if he's outside, then he's not inside" follows from the first premise. Even in such a case, however, the validity stems not from affirming the consequent, but from the form modus ponens.