I believe that's what the GP means, yes. With a future superheavy launch vehicle, however, a tandem mission could be possible, with separate Neptune and Uranus

I believe that's what the GP means, yes. With a future superheavy launch vehicle, however, a tandem mission could be possible, with separate Neptune and Uranus probes being inserted into different trajectories by the same LV/upper stage. Both spacecraft would still do a Jupiter slingshot but at different times.