I think the issue with Tyler Phommachanh is the lack of practice reps. Roberson did play in the spring, but Tyler Phommachanh did not as he was still recovering from injury. Turner and Millen didn't arrive at UConn until the summer and it appears Millen had an injury. Thus, I would think the vast majority if reps in camp went to Roberson and Turner with Millen and Tyler Phommachanh being brought along slowly as they were recovering from injury. After Roberson was injured in game 1, I'm sure Turner has taken the vast majority of snaps in practice as he was learning to be QB1.
And, Tyler Phommachanh's and Zion's stats look very similar as true freshmen:
Tyler Phommachanh: Passing: 35 for 72 48.6%, 321 yards, 1 TD, 2 INTs. Rushing: 24 for 92 yards, 2 TDs.
Zion: Passing: 44 for 86, 51.2%, 390 yards, 4 TDs, 3 INTs. Rushing: 22 for 60 yards, 0 TDs.