No, I am not saying that at all. I am saying that you will descend faster on a -5% grade than a -1% grade REGARDLESS of the trainer difficulty setting.
Are you talking about trainer difficulty or climb portal difficulty? Because they are two very different things and it sounds like you are mixing them up.
Trainer difficulty affects the amount of resistance you feel in a smart trainer. Set it to 50% and a 10% climb will feel like you are riding up a 5% climb but your speed will still be exactly the same because trainer difficulty does not affect your speed on Zwift at all.
Set climb portal difficulty to 50% though and not only will the climb feel easier but you will go faster because climb portal difficulty does affect your speed on Zwift.