The OD directs are supposed to hold in forward gears also, but, there is a sprag clutch in the OD unit that will hold on forward movement also. You lose the OD directs and sprag engages going forward but releases in reverse so slippage or just burnt out and it doesn't go reverse.
The OD directs only disengage in OD, that is the clutch pack that is applied with the 800 psi spring and is hydraulic release. The sprag clutch allows the OD directs to disengage first then the OD ltuch pack applys to get OD. This was a problem in the 518's and early 618's as the clearance on the OD clutch pack was too tight and would apply the OD clutches while the directs were still engaged. The puny OD clutches never stood a chance when that happened.
He did say, the way I read it, that the truck would move in reverse just not very fast and it would not accelerate. Any time I smoked a rear band, jammed or broke a rear servo, there was NO reverse. That may not hold true if the piston is slightly cracked or hanging at the top of the bore with the band partially applied. A pressur echekc of the rear servo should eliminate the servo being broke or sideways in the bore as the pressure is usually low when that happens.
Any time I have seen a direct clutch pack welded, and I mean well and truly welded to where you had to beat it apart, it moved in neutral from parisitic flow of fluid and loaded the engine in park. However, there are other things that could go wrong or break and it would not move in park either so it is not a hard and fast rule, just a general observation.