No. The Poster is not referring to
Hill Start Assist: A Feature when Stopped on an incline, in which the vehicle will apply the brakes so that you don't need to; this prevents the rollback that you would normally see in a manual transmission vehicle when you release the brake to hit the gas when starting on an incline.
Poster writes after he Parked vehicle, then starts up and shifts from Park to reverse. That's when he doesn't have immediate hold.
I never had any hold - with any vehicle in that situation. I'm surprised it even grabs at all. Cause by default you're tilting downward and also in gear reverse.
I have to try that sometime! As I've always had too press/hold brake while rolling down.
Sounds like you got a good thing