Again, this policy does exist, and has been confirmed by Delta PR in the past. It’s just unpublished, and therefore we don’t know if it applies to BE tickets or not. We can guess, but the best way to find out if this policy was applied correctly in this case, and to express disappointment if it was not or if BE tickets are excepted, is to contact Delta customer service, who are much better positioned than random passengers on FlyerTalk to know the details of the policy.

OP should expect that the response will be polite and with no compensation, but this situation is ambiguous enough because there are no public statements about how the Flat Tire rule applies to BE fares that it’s not unreasonable to ask. Remember, BE fares receive identical IROPS handling to Main Cabin fares, so it depends on how this sort of change is treated internally as to how it is expected to be handled.