Who hired Schwarz? Yeah, I think that was a Marrone decision, Schwarz didn't get hired on his own... and quite frankly wasn't a proven great DC before he got to Buffalo. He certainly was not a good HC.
Schwartz was only hired because Marrone's choice for DC -Pettine - was hired away. Under Schwartz, and without a key starter (Alonso) for the entire season, the defense was much improved from the Pettine D.
If you're going to praise Marrone for leading the Bills to a winning record, it's odd that you say that Schwartz was not a good HC. After all, Schwartz led the Lions to their first playoff berth in over a decade in 2011...