The reason nobody has used high-power electrostatic motors is that they require high electric fields, which would cause the electric breakdown of air and of most fluids. In contrast, the normal electromagnetic motors use high magnetic fields, which do not cause the breakdown of air, so they do not need immersion in an insulating fluid.
It is likely that the fluid used by them is some kind of fluorinated hydrocarbon, as those have high breakdown fields. Therefore leaks from such a motor are undesirable, so it would be interesting to know how do they prevent leaks between the rotating axle and its bearing. Rotating seals can never be perfect, as the users of Wankel motors must be aware. The main reliability problem of the Wankel motors has also been the rotating seals.
I assume that nobody has tried before to make such motors because nobody has found a way to prevent the leaks until now.
Perhaps the motors are intended to work only with the axle pointing upwards, in which case gravity would prevent the leaks.