I have seen a power supply many years ago using the same topology. It seemed nonsense to use 2 lower specced transistors then one higher, given the additional complexity. However the economics must have dictated this. Perhaps their inventory only included components for the
110/120V market where just the one was fitted?A full H-bridge gives you a greater transformer efficiency, as well as easy clamping of spikes to each rail.
To be fair for the design, there shouldn't be a route for positive spikes to be transferred to the input mains through the bridge rectifier. Power can only pass into the welder, not back out[1].
[1] I am aware there is a finite diode turn off time, but the energy associated with that should be pretty low.