I have no idea where you're getting your numbers from. Selecting that one time a ton of plates dropped doesn't really prove anything.

One thing you are right about is that the higher tiers are definitely better for farming HMP specifically, but people do those a lot less nowadays because they lose out on the chance for reforging mats which are also chances for gil anyway (and for someone actively working on a weapon, like you pointed out, gil = more plates).

HMPs off Pil and his partners is about 33%.
Pouches off Qilin and his partners is about 6%.
Morta and the like drop HMPs at about 15% if you're generous, and about 4% cinder and 2% dross (those two are inverted depending on the NM)

Pouches give 3~15 plates so even if you were in an alliance of 18 and every single pouch you got gave 15, you'd get like 97 plates on average given that 6% drop rate. Morta's a different story altogether since you nobody goes there for plates anyway.

I really think they should do something with all that voidwatch gear we slaved away to get, but I think recently they did say they'd like to move away from reforging stuff.