It does seem there was something vital that he didn't know or wasn't aware of. Perhaps he wasn't aware of the many denials of a quid pro quo that had been given, including by Trump. Or he assumed that in the context of the impeachment discussion, the term "quid pro quo", was used in a more narrow sense to refer to quid pro quos that are improper or illegal (which isn't the case either, but would require clarification anyway).
Otherwise how do you explain that he described very precisely and multiple times a quid pro quo situation, and not show the slightest indication of being aware that this was in an almost absurd contradiction to Trump's central claims? And, by the way, also in absurd contradiction to his later reversal claiming there was no quid pro quo. The contradiction was too straightforward, too obvious, too simple to be a result of momentary confusion. Or to be at the direction of Trump himself (I'd think).
Here is a Youtube video of Mulvaney's complete press briefing, the relevant part starts at 33:08 :