Never been proven. No repeatable experimental data to support it. Some people think that the technology was lifted from the remains of the Roswell UFO. It's not that important. I needed a way to move things around that didn't require fossil fuels. So you rotate a room-temperature superconducting magnet at high-speed and voila! electrogravity. In the book, electrogravity (EG) was only a repulsive force, not an attractive one. So it was only good for pushing against gravity. Works great at sea level. Not so good once you reach the upper atmosphere and useless outside of a gravity well.
As long as I had to build in a rotating superconducting magnet, I used it to clamp the space tugs to the body of the Ark II which was made of pig iron so it was magnetic. I also created some shields to cover the EG lifters so it "reflected" the repulsor field backwards and created artificial gravity. Rei and Rome needed this for their year-long journey to Deucado so their bodies did not atrophy.
I also had MINIMCOM use the EG lifters as a super-duper PA system upon occasion.
Very convenient for the story but unlikely to be too useful any time soon.