I noticed a JAL roundtrip that is somehow significantly cheaper when purchased through an AA codeshare. JAL has great mileage earning rates on Alaska, especially on premium cabins.
If purchased through AA, does this itinerary earn miles on Alaska at the JAL rate (good) or the AA rate (not so good)?
(I know Star Alliance earns based on the operating carrier, not marketing carrier. But not sure about this situation since Alaska is not even in oneworld.)
We can say for certain you do not qualify for the JAL/Alaska rate on a JAL-operated, AA-marketed flight. As stated on the very page you link (emphasis added),
Flights must fall within the eligible flight number range below, and your flight must be marketed and operated by Japan Airlines.
Eligible flight numbers 0001–8999
To ensure flight credit, the two-letter airline code (JL) must precede the flight number (for example, JL XXXX) on your ticket receipt or boarding pass.
According to Alaska's blog post on the 2018 changes (emphasis added), it seems you might be able to accrue on AA-coded international flights:
You’ll continue to earn Mileage Plan miles on all American Airlines flights internationally, including flights from the U.S. to Canada and Mexico, at the same rates that you earn on these flights today. Again, Mileage Plan elite members will continue to earn elite-qualifying miles and elite bonus miles on these flights. International flights will not have an Alaska Airlines flight number – they will all start with airline code “AA” and can be booked at aa.com
The scenario is not covered in the main FlyerTalk thread about the 2018 partnership changes. An earlier thread on crediting to Alaska on an AA-marketed, BA-operated flight (British Airways, like JAL, is both a oneworld partner of American and a bilateral partner of Alaska) suggests that you will need to submit boarding passes to Alaska and claim mileage credit after the fact.
