They used corner delta because the only load is the motor and a 240v MCC. There is no 120 available and it is not needed. The 4 wires are 2 hots, one neutral (another phase) and the grounding conductor. It looks visually exactly like 120/240 single phase except that the "neutral" is actually tied to the 3d phase, not the center tap of a transformer. In fact they did it with just 2 transformers similar to the way they do center tapped delta vee. (red leg delta) The advantage of corner delta to the installer is they can use cheaper
2 pole switch gear. The only added requirement is everything needs to be 240v rated (delta rated breakers)