> when the queue gets bigger than one. It looks like a second car would queue on the main road, blocking traffic.

Without the buffer, a single car wanting to turn that way when there is a cycle in the lane would block traffic, unless of course the car takes priority and just expects the cyclist to deal with them cutting in front (which is my experience too often at junctions with or without cycle lanes…). In either case, with or without this design, the car slowing down to turn is going to create some back pressure if the road is busy, there is no avoiding that and this design might even actually slightly reduce that issue.

Looking at the picture I assume that most vehicles are going to be going straight on, and when someone is turning the only extra delay is when their need to turn coincides with there being cyclists or pedestrians in range of crossing, so it is likely that none of this back pressure is a problem the vast majority of the time.