Flights booked on partners with partner coded flights earn MQDs based on distance (this was true prior to 2024 as well). A discount business ticket for ICN-GUM on KE goes for about $1000 roundtrip, but would earn roughly 1200 MQDs because it earns at 30% of distance flown (4000 miles) and you would actually earn more MQDs than if purchased as Delta coded flights at the same price. They have reduced the MQD earn on tier 1 partners in business class from 40% to 30% of mileage flown for 2024, but it is still possible to earn more MQD on partner coded flights than DL coded flights at times when the price is less than DL coded flights.

There can be cabotage issues with selling tickets on foreign flights that don't go through US. In this case, it may simply be due to a lack of fare filings (since GUM is a US territory). DL does not have any fare filings on ICN-GUM non-stop, so it can't sell you any fares with DL coded flights. When you purchase a ticket from JFK-GUM through ICN, you are purchasing a single through DL fare on JFK-GUM route (you are not purchasing separate DL fares for JFK-ICN and ICN-GUM). There are no cabotage issues with offering award tickets on routes (or requirements for revenue fare filings).