Perhaps true, I don't know, I could see it with tighter chain bend, possibly also chainline angulation. Depends on the friction in the chain. That said, most of the time, you are not in the high gear. My 20"x1.75" 50/34 x 11-30 cassette, I'm rarely on the 50x11/13. There can be reasons when an IGH is unavoidable, but I sure like to avoid them. And don't mistake, I consider IGHs to be modern miracles of mechanical engineering, they really are! The drawbacks for me are, in order, 1) cost of service (and cost of replacement if not regularly serviced, i.e., annual, if ridden in rain), 2) repairability in field, 3) repairability at home, 4) weight, 5) loss of efficiency in all gears but direct drive, 6) initial cost. Also note with regard to a Brompton: 7) Even with an IGH, it STILL requires a derailleur (in this case, chain tensioner) due to the swinging triangle geometry that slackens the chain), and 8) Extreme spoke angles due to large-flange rear hub on tiny 349 rims, that's a lot of spoke angulation, better if the rims are dimpled and aimed-drilled, which they're not. (And you can't go radial spoking, at least on the drive size.)

The drawbacks of external (derailleur) gearing? 1) Lateral wheel dish, especially critical on a 349 wheel, so the angles are even worse, 2) Skinny chain if you want a LOT of gears, so less durable, 3) Derailleur clearance to ground, tricky on a 20" with 1X gearing, though 9-36 may do it, SUPER tricky with that much cassette range on a 16"/349 wheel.