It is true however that Mangums stats would obviously be inflated against g5 teams. If you want to compare him to QB's of other G5 teams that is a fair comparison, but comparing his stats to a P5 teams qb's stats would be comparing apples to oranges.
For example Utah State has the 15th best passing defense in the Nation. Alabama has the 20th. Is that true? He** no its not true. What is true is that Utah State has the 15th best passing defensive #s in the nation playing their crap schedule. If they played Alababama's schedule they would be closer to 115th in the nation.
Look at BYU. They are ranked 56th in defending the pass. Does anyone really think that Micheal Davis and company are better defending the pass than TCU, Mississippi St, Tennessee, Baylor, Iowa, Auburn, Michigan St, Stanford etc etc?? Not a chance. Those teams statistics came against legit 9-10 game schedules and BYU's #'s are inflated going against cupcake state.