Not sure why he thinks only QBs can be successful OCs.
Debord was an OL/TE guy. Bert Beliema was not a QB. Neither was Fulmer. Dan Mullen played TE in college then coaches WRs and eventually randomly became QB coach, Jeff Scott/Elliot Clemson combo were WRs in college that coaches WRs and RBs, I'm sure there are more.
You probably prefer a QB guru to call plays but it doesn't mean they are more successful than a OL/TE guy.
Reasons to worry about Scott but it's not because he wasn't a QB. Worry because he is inexperienced play caller. Maybe he will use our TEs more