Oh, yes, I would expect the bus in June to be full with standees, especially west of Amalfi. The other problem is the crowding of the road, and the busses that can't make it around the turns in one try. I would observe that the bus ends in Amalfi, and the second bus (maybe Amalfi-Salerno, I've never been on it) would not be nearly as crowded. That could be a backup for a full or weather-socked-in ferry. Technically, these busses have luggage lockers under the bus, but you are right that they are jammed from Sorrento to Amalfi.
I would guess that a certain number of people get off in Positano. But I understand your concerns. (Recall Rick's advice to visit Positano first, because the late bus home to Sorrento may be so full from Amalfi loading, that it doesn't bother to stop in Positano!!!)