Yep that does seem to indicate he was told "You won't be the DC" and is simply moving to a place with more upward mobility possibilities.
I know he was a decent recruiter and I hear a lot of comments about how he was a good DL coach and that may be true. But as a casual observer watching the DL I didn't see a ton of growth or anything special out of those guys. Could simply be a talent issue but might also be that a guy who played RB only knows so much about playing DL.
I don't see it as a crippling loss on the staff personally.
Maybe we can get a new DC but also pull Brick in with a co-DC role or something.