They were operated by coaches, I don’t think it was too long ago they switched over.
The processing of passengers at the bus station is a real gripe of mine. I use the Buchan services quite regularly which rarely depart on time for this very reason. Frequently queues can be 15-20 deep and it’s excruciatingly slow at boarding time with large numbers buying tickets, not helped by drivers attending buses just a minute or two before the scheduled departure time (not a dig, there could be valid reasons). I do feel that for some services, such as Buchan, Deeside, Airport etc, that roving ticket sellers would speed up this process immensely. It used to be done at Ellon P&R and worked well.