They are not transformers, though. The coil/chokes are not galvanically isolated which makes them (more) efficient. Stepping down from 48V to 0.8V (with massive transient spikes) is generally way harder than doing it from 12V. So they may ended with multi step converters but that would mean more PC with more passives.
3.3V from 48V is a standard application for PoE. (12V intermediate is more common though.) The duty cycle does get a bit extreme. But yes, most step-down controllers can't cover both an 0.8V output voltage and 48-60V input voltage. (TI Webench gives me one - and only one - suggested circuit, using an LM5185. With an atrocious efficency estimate.)
You'd probably use an intermediate 12V rail especially since that means you just reuse the existing 0.8V regulator designs.
Aside the step down, the transients can be quite crazy, which might make the power consumption higher (due to load line) calibration. 48V fets would have much worse RDSon compared to lower voltage spec'd ones. So it does make sense no single smart power stage to have such transistors (presently).
There are other issues, too. 48V would fry the GPU for sure, 12V often time does not even with a single power stage failure.
In the end we are talking about a stupid design (seriously 6 conductors in parallel, no balancing, no positive preload, lag connectors, no crimping, no solder) and the attempted fix is a much more sophisticated PCB design and passives.
