But arriving at the airport 107 minutes before departure should still have been ample time to reach a bag drop machine well before the 75 minute cut-off.

(Not having to worry about bag cutoffs is one of the reasons I avoid checking bags when at all possible! The only cutoff I have to worry about if I can get a boarding pass is being at the gate at T-15 — which in reality is almost always possible even if you aren't at the bag drop at T-60, and it's so much less stressful to only have the one deadline to worry about.)