There's no advantage from what I could see, so it rules that out even though you'd instantly blow and not offer advantage unless he had an open net.
Ultimately yes, it's a stonewall penalty, not even a hard one if you just watch Mustafi all the way. I can only assume the ref's completely missed Mustafi and because the player has recovered almost instantly and got the ball that they came together mutually.
All that's done is encourage players that they have to flop and roll to get the decision, genius stuff.
I hate to be that guy, but the standard of refereeing has been getting more and more shocking as the years have went on. Yet we're all told by the FA to respect and whatnot. There's been some stinking cards handed out, some stinking challenges missed, my own team included in that which led to retrospective action instead, and a whole host of questionable decisions and we're just over a month in. I don't care anymore, for the love of god just bring Video assistance in fully fledged.