neither was Andy Reid ever an OC
The difference here being that Andy Reid made himself the playcaller when he became HC. He took the offense from Green Bay, put his own spin on it and has been a playcaller of it ever since.
Campbell did not do that. He briefly did some playcalling when he took away those duties from Lynn because it was going so bad. But Campbell is not a playcaller - which is fine as a HC. He's just more of a CEO style HC.
Now, having said that, I am sure that Campbell had input into the offense. It's his offense now in the sense that the Lions have the playbooks from the last 3 years and can continue to use them, iterate on them, etc. He can choose to keep the same terminology - that's easier to do if you promote from within. That's what Belichick mostly did. But everything I have seen points to Johnson being the brains behind that scheme - it's his scheme that he tailored with input from Campbell and Goff.
It's difficult to bring in an outside coordinator and then tell them they have to use their predecessor's playbook and terminology. The Bears actually tried that on the defensive side when Nagy was HC - they brought in Pagano to try and run Fangio's defense. It was not smooth and the defense took a definite step back.