I think he followed the guidance in the adventure and this is the first campaign for him using 2024 rules.
Also while it seems to be a lot, and it certainly feels like a lot playing in game, I don't think it is actually an abnormally large amount based on the guidance in the DMG. The DMG says 100 magic items for 18th level party, including 11 Legendary and 19 Very Rare. I don't think we actually have that many. We probably have more Rare than designed, but I think overall we are still in the ballpark for what is recommended.
We have 5 in our party, my PC currently has 2 Legendary items and 3 Very Rare items and I think that is about average for the party.