As you typed that you must have realized what an irrelevant line it was - Jennings had a phenomenal one off season and fair play to him for that. Scoring over 1200 runs at 46, better than you have ever performed before is hardly regression - its also a very good season. What more important is when you put that 1234 runs alongside the fact that Stoneman has been easily the most consistent batsman (in any position let alone opener in the CC in the last 5 years).
Since 2013 no English qualified cricketer has scored more runs than Stoneman in the CC - not one. No other English qualified player has scored 5 consecutive 1000 runs in a season - not one. Over that time hes scored way more runs at a better average than Trescothick and more runs at a better average than the likes of Robson, Lyth, Jennings, Compton and all the other England touted openers. But more importantly last year was comfortably his best year out of all of them so I ask you again how can he have regressed?