The buses are not shared with Pop Century.
The buses will be more crowded first thing in the morning, and at closing time of the parks when everyone is heading back, just like the other resorts.
For what it's worth, we took the bus from AK to the resort and back to grab lunch and check out the grounds last Saturday. We rode over at about noon, and we were the only ones on the bus. We rode back around 2:30, and we were the only ones on the bus. Both times at the AoA bus stop, there were no more than 20 people total waiting at the 5 different queues.