The list of items they can harvest doesn't include all the items you have harvested yourself. I was hoping my retainer could get some Lominsan Anchovies to make some more Fishmeal, but even though I've caught that fish myself, it's not on his list. He does have the right level gear on and mostly HQ. So I'm not sure what's going on here. So don't get excited over Fullmoon Sardines and Apkallu eggs, they may not be on his list.