The short unthreaded section is normal for the rotoflex shafts.
Although the hubs are not as tight to remove off the outer driveshaft as the Swing axle shafts, I still had to use a proper hub puller when I rebuilt the rear suspension on my Mk2 Vitesse
The threaded section tends to get stripped or damaged if you use the nut to pull the hub onto the shaft, a mate who helped me rebuild mine used a suitable brass shaft to drift the hub on before we tighten up the nut?
It was a faff setting up the end-float of the bearings if I remember correctly too, you need the slightest 1 to 1.5 Thou max play or you will ruin the bearings, anymore and chances are the M.O.T. Tester will fail the car too.
Gary