Huge difference between the idea of a cup of coffee and losing a given role due to poor performance. Cup of coffee has implications that just weren't accurate with Mullins, in neither 2018 or 2019. He played in 45 games in 2018, that is one honking huge cup or coffee.
I will also stand by ineptitude, he clearly showed a lack of skills or ability in 2019. He was on pace for a -5 rWAR season. That's demonstrating a lack of ability.