I can see your point, but;
It's an oil boiler so is more likely to be able to get hotter than 80C in a fault situation.
In the scenario of the pressure valve sticking, the cylinder could rupture. Something will give, that is for certain, flooding the house with (initially) very hot water.
The whole point of the G3 regs is to have two lines of defence if there is a problem. G3 was meant to ensure a potentially dangerous but useful domestic appliance can be safely and confidently installed by semi skilled people in a consistent manner.
Here we have an installer who, even though the manufacturer and the law have provided abundantly clear instructions, has, through their own negligence and lazyiness, chosen to ignore them. Sadly, I and many others in this industry see this type of install all to frequently.
And usually the customers tell us what a nice chap installed it.