Here's the official answer from TWAB last night.
Prestige raid lairs and armor from Solstice of Heroes were not intended to be infusion fodder for two reasons:
These items are intended to be trophies, for players to show their accomplishments leading up to Forsaken.
If players who own Curse of Osiris but do not own Warmind use these items for infusion, any gear infused above 335 Power will no longer be available for use.
Players who have already done this can claim a duplicate of their weapon from their Weapon Collections, which will become available to all players when Forsaken launches on September 4, 2018.]
Personally, I get the MoT armour. The cap is there and the event is to focus just on this new gear. However, the raid weapons at 400...well, that felt like it was just cos they didn't have time to build more new weapons so opted for an easy answer. Plua to push prestige raids, which imo still need more rewards.