Buses are never the only users of bus lanes. Even if cyclists, taxis and motorcyclists are not allowed (they are many places), there's still police cars, council vans and bus service vehicles using them which could be concealed by lorries. If your interpretation is correct and the driver was only looking for big buses, they had a very lucky escape that what hit them was only a cyclist.
Again, you seem to be answering a question that wasn't asked with your own brand of what-aboutery.
@straas asked "what if it had been a bus".
@PaulSB confirmed it would have been visible.
You responded by asking how that was possible and calling him rude.
I offered that there are situations whereby it is possible to see a bus but not a cyclist. It seems to me to be the best assessment of the facts given. No one mentioned, motorbikes, taxis, scooters, hoverbikes, landspeeders, police cars, ambulances, etc etc etc. For clarity, no one suggested that the driver was looking for buses, or indeed any vehicle at all.
What was offered is that buses in general are much bigger than bicycles and can therefore be seen over other vehicles. It may have been when the OP said "high sided" he wasn't making a forensic definition, just trying to convey the information that the cyclist couldn't be seen over the vehicles allowing the OP's son to pass through and then over the bus lane. The OP's son may or may not have had a lucky escape.It is however possible that he could see a bus and not a bike.