This is one thing I think Snyder is probably on the right track about, though as you say, perhaps not for the right reasons. I think offensive coaches are preferable to defensive ones because the NFL is a scoring league. The rules are mostly tilted to produce big offensive performances, especially in the passing game.
Therefore, teams like Buddy Ryan used to put together probably wouldn't win more than six or seven games in today's NFL. They'd be penalized too much for things like roughing the passer and P.I. which would in-turn remove what made those teams successful , i.e. forcing turnovers and scoring defensive TDs.
Another concern is the supply/demand issue. In general, it's not too difficult to find a decent to very good DC as there almost always seems to be a bunch of them around. However, finding good offensive minds that know how to call a game, develop game plans, get the concepts across to the QB and other offensive players and then turn all of that into points on the scoreboard is much harder.
Just my .02