you would think HKG and TPE would both be on the "possible" list. The advantage of SEA is that a 763, one of the lowest trip cost and smallest widebodies, can make do many SEA-Asia flights, so it is a good aircraft to develop the market.
DOT data shows that HKG from the east coast and midwest doesn't generate the level of revenues for US carriers (probably do to the strength of CX) that China does so it makes sense to serve HKG in the most efficient manner possible.
China and other markets in SE Asia are growing faster than HKG and DL does have Chinese partners but none in HKG.
Note that the DOT has stated in the decision that DL and AS' relationship even though not an immunized relationship is strong enough to provide the feed necessary to make the route work while also arguing that AA and UA continue to enjoy ATI w/ their Japanese partners. Thus, it appears that any well-developed business relationship between airlines is sufficient to achieve the objectives of providing market mass in the eyes of the DOT.