No, it's just a compromise solution. If you have a converter rated for 30hp of load, that has to be running almost idling all the time you want to use a small 3-phase device that can be wasteful. It does nothing to temper the possible heavy starting loads, just makes them bigger and transfers them to the single-phase supply. Then when running, the performance and efficiency of the load motors can be somewhat reduced. An inverter on the other hand can be finely tuned to get the most appropriate starting characteristics for its machine, with a minumum of surge load, and then allows the motor to work at peak efficiency. The main drawback is initial cost, and some motors are not configurable to run from a singe-phase-input inverter.
As I understand it the transformer feeding the site is single-phase, as is the 11kV overhead line. There might be more single-phase power available from the existing transformer, but to change to 3-phase requires an extra 11kV line and a new 3-phase transformer.