If OP can characterize the "snark" he can complain about that, although he himself says that he argued and then said that he wouldn't.
DL does not reserve OH space and there is no entitlement to it. Makes sense to keep the bags which fit under the seat under the seat until it's clear that there is space in the OH so that fewer bags are gate-checked. Delaying the flight at the last minute is a poor option.
Not relevant directly to OP's post because there was room in the end, but it would be really helpful if DL GA's strictly enforced the carry-on limits. Never a guarantee that everybody gets their compliant carry-on in the cabin, but makes it much more likely and it would give experienced travelers the comfort to board towards the end of the boarding process.