I think this is a significant hire for a few reasons:
1) He's recognized as being a solid coach. Name was out there for other positions.
2) In particular, he was linked as the Jets' likely next Def Coord. But Jets couldn't land him.
3) Sounds like Leonhard OPTED for Bills over Jets. Jets could have used his help, seeing that Jets had ZERO ints this year.
...One side-bar on hires like this. a) I still can't believe Glenn retained his job w/ Jets. b) There's a CHANCE that Glenn rebuffed Leonhard. Hot-seat HCs like Glenn don't like bringing in a solid Coordinator who be seen as the Head Coach's replacement if/when the team struggles.