I think having Ndombele and Lo Celso in MF with him instead of Ward-Prowse/Armstrong/Romeu has made a big difference. Also Toby instead of whatever CB they had helps as well. His biggest issue at Southampton (and I would say still here) is that under pressure he was poor, with Southampton he had poor outlets to use so it resulted in a lot of turnovers. Here he can use the better outlets so that the bad turnovers are not there any more, and if he is forced to make a bad/tough pass to someone like Toby, Gio or Ndombele instead of giving it up for a quality chance like the what happened at Southampton, they can deal with it to help him out.
I know it sounds simple to say that playing with better players makes you better, but for some it isn't the case as they stand out for how bad they are, or for some it raises them but only slightly. For PEH I think he is a guy that is a lot better playing with top level players.
Also there was a lot of sour grapes from Southampton fans whether they want to admit it or not.