I don't know everything about Pelton's model, but my intuition says that 1) Pelton's model is probably underrating Tre's shooting and 2) Tre is a player that models will just generally dislike.
1) Pelton seems to project shooting based on the standard shooting indicators (3FG%, volume, FT%). Tre's numbers are good, but his volume is high, but not out of this world. If you looked at his tankathon 3FG% projection which uses the same factor, it's very good but also comparable to very good shooters in previous drafts. What separates Tre from the others is that the degree of difficulty of his shots seems higher. That is really only caught in 3FG volume, but his volume is not as high as his degree of difficulty might suggest. So he's probably a little underrated in that aspect.
He also incorporates high school 3FG% which wasn't great. I don't have great opinions on how much that factors in or how much it should (if at all).
2) Even if we are certain about Tre's shooting, we must remember that shooting is one of the flimsier things to predict. A model can never be too sure about shooting because it's generally hard to predict, so when a large part of player's case is based on shooting it can make a large part of his case feel unpredictable. This doesn't necessarily seem to be the case with Pelton's model, but I think that's something you see generally with those who attempt to model the draft. Beyond that, his peripheral numbers are extremely low and those are pretty much universally agreed upon as big indicators across the board model wise. So Tre was never likely to be a top 15 player in this context.