So the context of the rather raucous press conference was that Mulvaney wanted to say that the overall reason for aid holdup was corruption in Ukraine and Europeans not contributing. Rather lame of course because all that should have been happening before Congress approved the air. But maybe it is a brinksmanship game, and Mulvaney was making that point. But he committed an unforced error. From the White House transcript:

Q But to be clear, what you just described is a quid pro quo. It is: Funding will not flow unless the investigation into the Democratic server happens as well.

MR. MULVANEY: We do that all the time with foreign policy. We were holding money at the same time for — what was it? The Northern Triangle countries. We were holding up aid at the Northern Triangle countries so that they would change their policies on immigration.

So he clearly agreed (previous sentences make clear this was no misunderstanding, or misspeaking) that an investigation into the Democratic server was one of the conditions for letting the military aid flow. So without some link from corruption in Ukraine to the Democratic server (which to my knowledge there is none… nor even any link of server to Ukraine) then Mulvaney indeed flubbed it.