Yes, I thought I remembered a YouTube video / interview a few years back with someone from LQMT where this was suggested in the interview and seemed like sort of a hint. It never really got discussed a lot on here though if I remember right.
perfect summery jrs5 and its backed up by the rumor the antenna needs internal space having it on one side/edge of the external frame won’t work thru just 1 cutout no room to have 4 antennas for each side and it would be too costly/power drain. the internal mid frame also needs to be rf transparent for the mmwave antenna. as payMEmf says demetallization. https://www.fastcompany.com/90546391/source-only-one-of-apples-new-iphones-supports-the-fastest-5g