The delay was designated "enroute ATC only."

If the flight were payload optimized, here are a few things to keep in mind:

Passengers will never be involuntarily denied boarding.

If bags were pulled ***because of weight*** then cargo must have been pulled first.

I think the FA said "payload optimized" without actually knowing what that means.

Flights are designated payload optimized early in the planning process. Since the delay was enroute ATC I think the FAA told Delta to kick off weight or some part of the plane was inop.

When flights are known to have cargo far in advance, seat capacity is restricted. On one seasonal route that I work, an A320 (F12/Y138) would only sell F12/Y120 due to consistent cargo expectations. Since the seat cap were restricted so far in advance, an oversell would cause solicitation for volunteers before cargo is pulled. This is because the flight is not payload optimized since cargo was always planned for it. OP sounds like his flight either had unexpected cargo or something broke.