Yes, it is residual current detection, what the rest of the world calls RCD. GFCI is a very specific term for a type of low level tripping Ground Fault protection circuits in bathrooms and kitchens that would be way too sensitive for something like a VFD or anything else running an industrial motor, but our equivalent to an RCD is GFP, equipment ground fault protection, also sometimes called "arcing ground fault protection" (but again, not to be confused with AFCI)
So as said, the VFD looks at the current from each phase and sums them up, expecting that everything that goes out comes back and nets out to be equal. Anything that doesn't come back, meaning the net sum has a residual value, must therefore be going to ground.
The reason that an across-the-line starter doesn't trip is likely because the leakage to ground is at such a high resistance that the amount of current flow is not seen by anything in a simplistic cheap starter. But a good solid state OL relay would see it, or a GFP device. But people often don't spend the few extra dollars those cost, so the leakage is allowed to continue. Sometimes that can go on for years, but usually it just slowly gets worse until the motor smokes. The VFD is just better at protecting the motor than a simple X-line starter is.