You think an EF to RF teleconverter would not add that length? I'm no optics engineer, but there would still be the flange distance to deal with. That isn't going away. Who knows? Maybe it is possible. I don't think so though.
No, you got it. The optical formula for every lens has a focal point, where the light converges and that's where the sensor has to be. In order to make an EF lens work on an RF body, whether you have a teleconverter or not, you need to add a spacer to increase the distance between the sensor and the lens (since RF has a shorter focal flange distance). There's no way out of it without a different optical formula. The only time you can shorten the lens without any other changes is if there was a bunch of space between the first glass element and the lens mount anyways.
If Canon made an EF to RF adapter with a built in teleconverter, it would be exactly the size of an EF to RF adapter attached to a teleconverter.