Often1

No need to bash the GA until the facts are in. The OP is so full of inconsistencies that it is hard to attribute rudeness. By the OP's own report, the only potentially rude thing she said was that "it may work that way on American and United." That is hardly seriously rude and the only way I can see a DL employee mentioning another carrier in this context is if OP raised it.

For what it is worth, it is not only DL, but AA, UA and with some limited exceptions, all carriers, that if you no show for a segment, that the entire ticket is cancelled. Unless the phone agent cancelled OP's ticket and then reissued it as ATL-DCA only, the ticket would have been auto-cancelled by DL's systems at T-15 from scheduled departure. I presume that with Irma closing in, DL had a lot of standbys and simply popped the first two on the list into OP's seats.

OP needs to rewrite his post accurately and with a timeline that makes sense. Otherwise this is just one of those things which could not have happened the way he says it did.