That's more to do with the reduced density of the Asian widebody planes - the JL 789 with Apex has 1 less seat than the AA 321neo with many boarding benefits:

• 2 aisles to use rather than 1: Can board twice as fast, even faster when factoring in the middle section people in W/Y who can pick the shorter aisle when boarding and switch aisles if there is an obstruction in their selected aisle.
• The seats also have more legroom than AA planes and there is higher ceiling clearance in the seat area vs narrowbody planes, allowing a larger percentage of people to stand in the seat section rather than the aisle to get stuff out of their luggage before putting it in the overhead
• Finally, window seats only have 1 person between them and the aisle rather than 2 on the 737/320s, so less time for the line waiting for already-sitting people to stand up and get into the aisle so a window seat passenger can sit down.