Lol what? The Head Coach absolutely has full and complete authority over both offense and defense, regardless of his background. There may be situations in some programs where a defensive head coach hires a very good, seasoned OC who has full control over personnel, playcalling, strategy, etc. but Brian Lindgren is not part of that group. It has definitely been Mac's decision to leave Sefo and Montez in the game far longer than they should have and not give the backups those reps.
I will say this about these kinds of decisions: When a coach is not very confident in the team's ability to close out a game, or are worried that one or two big mistakes could make a blowout a close game, coaches will general err on the side of leaving their starters (at any position) in the game longer than they should, rather than pulling them too soon. The absolute last thing you want to have happen is to pull your starters, get them comfortable on the sideline and out of the playing state of mind, and then have to reinsert them back into the game when plays need to be made to close it out and the other team has the momentum.