I used the term coach builder to differentiate it from the chassis manufacture and it relates to the company that builds the body of the caravan/ motorhome, or any superstructure not supplied by the chassis manufacture.
It is a common enough reference used throughout the motor vehicle and trailer industry.
I agree the site I pointed to is not a government site, but it is far from the noddy type organisation you casually accuse it of being. It is a well respected organisation that lobby's on behalf of the trailer industry, and as such it is highly knowledgeable on the legalities of towing.
I would most definitely believe the accuracy of its web site over causal commentators on a forum.
Just out of interest the Camping & Caravanning Club
https://www.campingandcaravanningclub.co.uk/helpandadvice/technicalhelp/datasheets/matching-car-caravan/
state
"The standard MTPLM figure for caravans given by manufacturers is calculated according to the British and European standard BS EN 1645. It's technically known as the MTPLM (Lower Limit) and in some cases this can be increased to the MTPLM (Upper Limit). This Upper Limit is based on the physical limits of the caravan itself, such as its chassis and axle limits. If you have a suitable towcar and your driving licence allows it (Data Sheet 40 - driving licences has detailed information about the limitations of a licence), you can ask your caravan dealer to - replate - the unit to the higher MTPLM with the resulting increase being the amount of luggage and other extras you can carry. It's important to do this because if you are challenged by the police or insurance company and questioned about the legality of your outfit, it's the figures given on the plates and accompanying documentation that will be used to validate your statements."
The NCC state
"Maximum Technically Permissible Laden Mass (MTPLM)
(Maximum Authorised Mass)
As stated by the caravan manufacturer on the caravan weight plate (usually mounted close to the entrance door) – the absolute maximum weight that the caravan must not exceed to be legal on the road. It includes the allowances for the user payload – all fluids (water etc) and personal belongings that you may wish to carry (clothes, food etc)."
I presume you consider both these astute organisations make up answers from thin air also.