It is. Part of it was his quick release and his willingness to throw the ball away. The other part was Shula would have your hide if you let Marino get sacked. But if you were terrible at run blocking, he didn't care as long as you protected Marino. The reliance on the pass also led to poor possession time and that wore out the defense. Shula sacrificed everything for Marino, and that worked up to a point. He could always get you into the playoffs with his Marino strategy, but never win it all. Marino was allowed to be a prima donna. I once saw him stand next to a fumbled ball, pointing at it, waiting for someone else to recover. He spent a good portion of his sideline time screaming at other players.
I don't think Marino would have done nearly as well under any other coach. Shula got the best out of his QB's. Earl Morrall got the majority of starts during the perfect season. Griese did well in spite of not having the greatest passing statistics. Shula got David Woodley to the Super Bowl, too.
Marino may have been the greatest passer early in his career, but he couldn't or wouldn't run and scramble. Later in his career he couldn't throw deep, and it's a good thing he didn't play for Jimmy Johnson early in his career rather than the end.
I've never been one to hold it against a QB if he couldn't scramble. That's a bit of a "bonus" skill in football ... sort of like a pitcher who has a great pick-off move to first base and doesn't give up a lot of stolen bases. It's a good skill to have, but nobody is going to complain about a Nolan Ryan or a Sandy Koufax or a Walter Johnson if they don't hold runners on base very well. Just look at how totally immobile some of the top QBs in NFL history have been (Dan Fouts and Troy Aikmen are great examples of this).