Thread isn’t about where to earth the pipe, it’s the isolate the boiler part.
my house, 13 years old had the heating isolator next to the programmer in the utility room.... the boiler was through the wall in the garage. At the time there was no adjoining door, so if boiler engineer turned everything off at this isolator, he had to trust we wouldn’t turn it back on. (Or take the fuse away or whatever)
One point of isolation is less confusing, even if that is the mcb in the board.... which can be locked off