If Han was no threat, then how come Vader had to block his shots? Of course he was a threat, if Han hit that life support panel on his chest he would have been killed.
Han was no threat because
Vader could block his shots. He didn't pull his lightsaber because he didn't need
to. If Han was any threat at all, there would have been a dozen storm troopers behind that door instead of Vader and Fett (who did nothing but watch).