Eh.... I mean... you gave up your confirmed first class seat and volunteered to take a later flight for poketing $400. Volunteering for a later flight in the day comes with all the additional risks like rolling delays, weather changes, IDB, killer hornets etc. etc... I don't know why the gate agent thought there were many empty seats to accommodate everyone who volunteered, but they clearly didn't do their due deligence in confirming it. You can always write a complaint to Delta, but not sure what you will get out of.. maybe 5000 sky pesos, if that.