The rules are written thus to cover all eventualities; so that a design engineer for a manufacturer can design a gas appliance without being Gas Safe Registered, for instance.
Those boilers that we screw on the wall aren't designed by Gas Safe Register licence holders, and they aren't plumbed in design labs by Gas Safe people.
In terms of doing work in factories, offices and domestic buildings there are rules that have been proved and exercised in Law. In other words, whatever the person currently calling himself Harrison thinks, people are successfully prosecuted every year for doing as he advises. Many of those prosecuted were ex Corgi or Gas Safe registered, so could prove that they had, at some point, the knowledge.
Nevertheless they are still successfully prosecuted. And the HSE will continue to have people fined and locked up.
Mr Harrison can interpret the Law how he sees fit, but he is being irresponsible with ignorant impressionable people who may be swayed by his rhetoric. Ignorance is no excuse in Law. It is OK on an internet forum.