I believe (and I speak as an ex-boatbuilder myself who had to deal with this all the time) that there may be some confusion being generated here by the strange habit we seem to have in the canal world of buying a shell from one place and getting it fitted out by another, unlinked, business.
If a "boatbuilder" supplies a completed craft, there is absolutely no doubt whatsoever that the RCD must be complied with and the HIN is a part of that. If a "boat fitter" contracts with a customer to fit out a shell supplied by the customer, and acquired from elsewhere, then technically that will fall under the "self build" regulations; RCD and thus HIN is not required if the boat is not sold within 5 years.
Many people will, however, take the (imho) view that to have the entire process overseen and documented by a qualified surveyor, which will allow the owner to be sure that the new boat complies with the RCD and to then obtain a HIN for the boat post construction.
Sadly, in the discussion above, it is quite possible for everyone to be correct!