Reminds me of that time people were hyping up players like Sahin, Bender, Kagawa, Piscszek, Subotic, Schmelzer and probably half a dozen more at Dortmund as top class when Klopp was still there. I'd pretty much bet my left n*t that most of these Liverpool players (bar Alisson and VVD, maybe also Mané and Fabinh as a stretch) wouldn't be performing at half this level for any other manager in any other team. Klopp is no magician and he surely can't turn every Sunday league player into world beaters, but there are certain types of players that absolutely smash it for his sides and his sides only. Henderson is most probably one of those too, IMO.
The thing is that Klopp had the opposite effect on Henderson for a long time.
He turned him from a box to box midfielder who knew how to pick a forward pass before he received it and always moved well to receive the ball, into a holding player with zero press resistance, always passing it back to the centrebacks and otherwise jogging around in marked positions. He improved him defensively a fair bit but he regressed as a player overall.
It's only from December 2018 when he was moved back as an 8 (Klopp didn't believe he could play that role), that he started looking like the old Hendo on the ball, albeit a defensively stronger one with a lot more maturity and leadership to his game.
When Fabinho got injured and he moved back to the holding role, he kept his positive use of the ball and stepped up another level. He's even eradicated his most classical weakness of not taking the ball on the turn under pressure.
He's a really complete midfielder now, as comfortable holding the midfield as he is box to box, and does a lot to maintain our tempo with constant crisp quick forward passing, basically as good as the best case I ever hoped for him. But a fair bit of his progression to this point has actually been in spite of Klopp.