I think you've also ignored gradient and weight... gradient being the the whole point of the discussion
I think I get it though, the gearing thing makes sense. Basically instead of ramping up resistance, it just lowers the speed that is created from the power you're making, as a trade off. (Effectively lengthening it, as I said)
The rear mech cable on my road bike snapped a while ago, so by adjusting the high/low limit screws on my rear mech, it is fixed on my 5th smallest sprocket (which by default is 16T on my 11-32 cassette). So all I have is 34/16 or 50/16 as gear choices, which is no issue for ERG training sessions and can be made to work up any Zwift routes by adjusting "trainer difficulty" (slope feel).
I entered stage 2 of Tour de Zwift yesterday, doing the "event only" Innsbruck KOM After Party route, which officially ends at the summit of the cat2 climb after three laps around the town and up the little "leg snapper" incline. I'd already done my ~75min "TT Tune-Up" plan workout and cool down a few hours earlier, plus I had a fairly hard turbo day on Weds, so I lowered "trainer difficulty" to 40% in the hope I could complete the route without wrecking my legs too much and making a hard session today possible.
"Leg snapper" incline is ~7% gradient, but the power I needed to sustain climbing it in either 34/16 (lap 1) or 50/16 (lap2) was as if I was climbing a ~2.8% incline, ~155W
https://www.strava.com/activities/3017042392/analysis/393/566 /207W
https://www.strava.com/activities/3017042392/analysis/1291/1412 respectively.
Innsbruck KOM cat2 is ~5% average (but it hits over 10% and practically levels out in parts), but it rode on average as if I was climbing a ~2% hill, which including a silly little sprint at the summit averaged 200W while staying in 50/16 the whole climb
https://www.strava.com/activities/3017042392/analysis/2935/4847 .
In comparison, when I had working gears and I used 100% "trainer difficulty" as per normal up the KOM last Feb, I would have almost certainly stayed in the 34T ring and switched between various sprockets up to and including 16T to average 292W
https://www.strava.com/activities/2153525352/analysis/457/1849
100% trainer difficulty makes Zwift replicate the effort of a similar outdoor incline as closely as it can. However, this is limited by what slope emulation your turbo can do, for example my Direto can emulate up to 14% gradient. The maximum gradient in Zwift is ~20% (Yorkshire), so if I wanted to feel the extra gradient on all slopes I would set trainer difficulty to ~70%, at 100% all the >14% slopes feel no harder. But I prefer, when I have workign gears, to usually use 100% in readiness to tackle real outdoor hills and have a better idea of what gear I can turn on a given gradient on a good day.