Basically what they are saying is that it would take you the same amount of watts and time to climb the virtual hill that it would the real hill. It’s just that you are putting out the exact same amount of effort in different gears.
All that seems to matter in Zwift is watts. How fast your rear wheel is spinning is irrelevant. So say it takes you 5 minutes to climb a hill while putting out 250w on average in Zwift. Then you go climb the same hill in real life. If you again average 250w, it’ll take the same amount of time. So it’s the same amount of time for the same effort.
What’s the difference then? You just happen to be in different gears. Since you are “feeling” less slope in Zwift, it just means that you are spinning your wheel faster to hit 250w. So to give a 250w hill effort in Zwift, you’ll be in a faster gear compared to doing the same 250w hill effort in real life. But it will feel the same and take the same amount of time.
At least, that’s how I understand things to be. Someone please correct me if I’m wrong.