I can't see how HST's could have been regarded as multiple units as they can't work in multiple in the traditional sense of two units being joined together.
No different to a Voyager or Pendalino or any of the multitude of modern slopy-ended EMU/DMU 3/4/5-coach stock that has no corridor connection.
MU=At least semi-permanently-coupled stock (carriages and motive power) where the loco is not removed routinely during or at the end of each journey or end of each day and stabled separately and where the rake of carriages are not routinely broken-up.
We all know though that this is a muddy area, so I know what you mean.