If you crash, the weight of the adult (multiplied because of the crash forces) will crush the child against the seatbelt, which can cause fatal injuries. It's more dangerous than them being unrestrained. Seatbelts must only ever be worn by one person.
For the person who said it's "batshit advice" to take a baby on a coach ever (!) I think this is down to personal risk tolerance. The chances of an accident are not zero, but they are extremely low. If you prefer to get a private transfer so you can use a car seat, that's fair, but it's not at all batshit to take a child on a bus or coach with no restraint, people do it every day.
There is not literally a protective layer, that could be conjuring up a misleading image. What I believe the poster who said that is referring to is more of a combination of the fact that a coach is extremely heavy, meaning it will decelerate much more slowly even in a crash situation. That makes it less dangerous for the occupants. Stopping suddenly is what is dangerous in a crash. The other thing which is protective about the size and weight of a coach is that the chances are if a coach crashes it will be against something much smaller and lighter. This transfers more of the impact to the smaller, lighter vehicle.
Personally I've actually experienced this phenomenon from both sides, fortunately at extremely low speeds. I've been in a car which was involved in a side impact with a HGV at about 5mph - the lorry was pulling out of a petrol station and we were pulling out of a parking space. The car was written off and my mum had to be cut out of it. She suffered whiplash, luckily my sister and I were OK, just cuts and bruises. As far as I know the lorry driver was OK.
I've also been in a tram which crashed into a van which had disregarded a traffic light. The tram was only just starting to move, so the impact was fairly light but the passengers barely felt anything - nobody even fell out of their seat, a couple of standing people stumbled slightly. We were all able to walk off perfectly fine. DS2 was a toddler at the time, I can't remember if he was in his buggy or sat next to me on the seat, he was also fine. Again van completely written off (driver was OK though thankfully). The tram barely even had a scratch on the paint job.
If the travel is somewhere involving mountain roads or other steep cliffs then I might make a different risk assessment than I would for motorway travel, which is also generally very safe because everybody is going in the same direction.