So the OP was told something was new Delta policy. Knowing how front line people sometimes don't know the rules as well as we do but also knowing that Delta does change the rules with no notice (and often no notification) they went to a higher lever person -- who in theory should be more knowledgable -- to inquire as the current state of the rules.

That higher level person at that point was unnecessarily rude to the OP, who then escalated to a supervisor.

I don't see that as unreasonable, but this is FT, so of course they are fair game...