The issue arising here is that DL has a codeshare on the AS flights ANC-SEA, and operates SEA-Hub-TPA with its own metal and/or regional carriers. AS operates flights from ANC to the DL hub, but does not have codeshares on flighst from the hub to TPA. I see this a lot where DL will be selling a ticket SEA-MSP-Somewhere with SEA-MSP operated by AS, but AS won't sell the same routing, at least not at a low price.

The benefit here is that AS will ticket domestic and some international itineraries on other carriers as long as you have at least one AS-marketed, but not necessarily operated, flight. For example, AS could ticket something like BLI-SEA-IAD-MCO-DEN-BOI with BLI-SEA operated by QX and all other flights operated by UA/CO. You'd have to call AS to do this, but it is possible.

Your best option is to fly on a combination of AS and DL fares, though this might end up costing you more money. AS should also be able to sell you the DL fare ANC-SEA-Hub-TPA if you include an AS fare on the ticket. That said, the agents I've talked to have had a hard time finding AA/DL-marketed, AS operated codeshares, let along getting them to price out correctly.