The difficulty setting does NOT make the hills easier, it gives you a lower range of shifting. 15% is 15%. If you lower the setting like I did I could spin on a 15% grade in my lowest actual gears. In real life though you may be going so slow as to stall, but you never stall in Zwift because you are on a trainer. At one point I was doing 2mph. So, is that realistic? Maybe not. But from an effort standpoint it is still the same. When my butt started to hurt I stood and had to go to my big ring so I had enough resistance to stand. At that point I was around 250 watts, which I couldn't maintain anyway. I'm 69 years old and there's only so much I can do. That I do it at all surprises me sometimes. I had the setting at 25%. If I had it at 50% I'd have put it in a lower gear to get the same overall resistance/watts while standing. If at 100% lower gear still. But at 100% I'd have run out of low gears to spin and that would make it impossible for me.
10 years ago I attempted to climb Mt Haleakala in Maui, 10,000' of climbing over 36 miles. I had to stop at around 5800' not because I was too tired, although I was, I had just run out of gears. My low gear was a 34/28, which was lower than my own bike, but still not low enough. I'd have liked to have had a 34/34 so I could spin out where the grade had lessened enough to do that as a recovery point. 36 miles of continuous uphill is very hard even if it only goes to around 8% at points. When I attempted Haleakala in Rouvy I couldn't even make it up the first section. Something about Rouvy always seemed off, but that was years ago since I tried it.
The choice with Zwift is to have a bike with the gearing you ride with outside, or the gearing you might have if you are regularly doing 15% grades over a long distance. I would never even attempt a 15% grade to the radio tower, especially after already having climbed the KOM. I'd take one look and say fuhgeddaboudit. I was already close to the bottom of the list for the KOM.
Anyway, it is what it is. You have a choice.