If they weren't considered to be multiple units, why would BR originally have classified them that way? The APT was designated Class 370, and all the passenger cars were unpowered.
I really don't see why a unit has to be able to couple up to, and work in multiple with another unit before it can be classed as a multiple unit - surely the whole point is that it's a single train composed of multiple, (semi) permanently coupled "units", as opposed to the traditional method of making up a train with a locomotive and individual carriages.
Referring to my Platform 5 "Motive Power Pocket Book" from 1990:
The HST power cars may well be numbered as locomotives these days, but they're hardly traditional locomotives in the sense that they really only have one purpose; to be at each end of a train composed of carriages which aren't compatible with "true" locomotive-hauled stock.