Unless you can provide a link to some legislation or case law to confirm this, I have to say I'm sure that's an urban myth. There are only very limited situations under which road risk insurance can be invalidated, and this isn't one of them.
He’s talking rubbish, route registration even allows trips that are off route in exceptional circumstances. Road closures for accidents or temporary roadworks for example.
I spent last summer assisting with day to day operations at a friends bus company with him teaching me the ropes, his insurance covers him exactly the same as any other motor vehicle, to go wherever he wants. His public liability also covers all public from start to end of the policy dates.
Buses can reverse, but a risk assessment should have been made, I.e. the driver checking it’s safe before hand. I drive vans with poor rear visibility, before any reverse manouevre even we are required to carry out a dynamic risk assessment... I.e is it safe, can I see the full reversal in my mirror. Reversing round bends or out onto another road are big no’s, but reversing into a road is ok. If you met me on a lane and your in a car, me in my van, the car is reversing unless it’s straight back for the van, due to the visibility, and safety aspects, I can’t see if there is a for example a bike or pedestrian, or even a narrow car behind me, a car has its rear windows and can see those obstructions. I assume the same requirements would be for buses. It’s covered in health and safety documentation I was made to read.