I think the schematic you posted use analog mode ?
that means all series transistors share the same voltage drop ?
I also own a fluke 343A i think i recall it is 1100V at 25mA = 27 watt output
you have also seen the range switch and first voltage dial handle input voltage to serie regulator,
so the voltage drop is not too high, when only low voltage out is needed, quite smart.
what I claim this Twintex does :
they have one set of transistors made by a double pair, for better power handeling
they are the only ones in analog mode = handle the voltage regulation.
all other transistors are on/ off only, and in series, with their own bridge rectifier, also shown quite well in the video,
and when in off that stage is skipped by a reverse diode,
and when on, it simply add 100V to the input side, of the linear series regulator set.
quite smart way to avoid relay switching, so they use transistor stage switching,
sorry i dont have a schematic, hope one is leaked one day ??