As a 19 year old in his first year of college football as a starter, Huntley's stats against P12 opponents:

63%, 260-270 yards/game, 2+ TD/g, .7 INT/g

I don't know when Mangum was healthy and when he wasn't, but his stats in 2017, as a 24 year old in his 3rd year getting first time reps, against LSU, Utah, Boise, MSU (like a PAC12 schedule):

55%, 145 yd/g, 0.5 TD/g, 1.75 INT/g

Now I know a big part of this falls on BYU's offense as a whole and Detmer so let's look at Mangum's 1st year as a starter as a 22 year old against Nebraska, Boise, UCLA, Michigan, Missouri and Utah:

54%, 213 yd/g, 1.17 TD, 1 INT

I am not sure how this is even close. Mangum might be a more prototypical pro QB and hence be higher on draft boards (sort of like Heaps), but in terms of college production, it isn't really close.

Caveat: I absolutely omitted Huntley's worse game (ASU) when he came back from injury and clearly wasn't healthy.