He was engaged with a defender the whole time, and when they separated, he moved backward toward the line of scrimmage.

I don't even understand why that penalty is on the books. It does not fit with the NFL's whole offensive promotion agenda, nor does having a blocker downfield impart any real advantage in the passing game. At most, it distracts the quarterback and creates situations where he might throw to an ineligible man by mistake. Judging by highlight reels, people like to see sacks and downfield completions and broken field runs. An interior lineman downfield does not prevent either of the former, and enhances the possibility of the latter, in the case of a screen pass or run-pass option play.