The output voltages and A/C cycles per second are variable in traction output inverters. the output voltage and phase cycles change depending on load and speed factors.
I don't know who said it was 'impossible', but varying voltages to a 3 phase A/C motor is very possible, and the primary differences between a forklift speed controlled motor and a generally available 3 phase A/C motor would be the feedback to the controller/inverter provided by the encoder bearing.