That was because it was Europa League, which is full of mediocre teams- he has no experience with top flight squads. Bayern's ageing squad isn't compatible with Sarri's fast-flowing attacking football.
Perhaps with Barca, but they wouldn't go down to that level of Sarri especially with Valverde who's decent enough. Makes me question why a club like ours should even get a bum like him in the first place.
And btw, winning one Europa League does not mean that he has 'mastered' winning. That's literally his only major achievement, if that.