There's no one single power curve for Fliiiight. Because your wheel is part of your resistance unit, it varies from wheel to wheel.
Above: An example of a power curve for Fliiiight (speed in km/h)
Understanding Fliiiight's power
Each time you double your wheel speed, you double the maximum resistance Fliiiight can create and quadruple your maximum power.
This gives Fliiiight a power curve with low maximum watts at the slowest wheel speeds, but high maximum watts at relatively high wheel speeds. Your bike's higher gears let you take advantage of this, by allowing you to spin your wheel faster (increasing Fliiiight's maximum resistance and power) while maintaining a reasonable cadence.
Fliiiight's signature zero-noise, zero-contact resistance is created when the aluminum in your wheel rim passes through a magnetic field, generating eddy currents that resist the wheel's motion. Doubling your wheel speed doubles the rate at which eddy currents are created, which doubles maximum resistance.
Speed x Resistance = Power
Therefore, doubling your wheel speed quadruples Fliiiight's maximum power.
If resistance increases with speed, does this make it difficult to speed up on Fliiiight?
Nope! Doubling wheel speed quadruples the maximum possible power, and doubles maximum possible resistance. However the magnets' distance from the wheel also has an effect on resistance. Your trainer moves the magnets in and out, ensuring you maintain the target power (Erg mode) or resistance (Sim mode) set by your controlling app at your current wheel speed.
How can Fliiiight have a lower maximum grade than some trainers, even if the maximum power is far higher?
In real life, the steepest hills will require you to change to your lowest gears, and often slow your cadence. Therefore, maximum hill gradient is defined by the resistance the trainer can provide at a very slow wheel speeds. Fliiiight's maximum resistance per unit of wheel speed is limited by the amount of aluminium in a bike wheel.
On the other hand it's maximum power, assuming a high wheel speed, is high.
What does this mean for my training in ERG mode?
In ERG mode you should use the gears that allow you to spin your wheel the fastest, so your maximum resistance will be high.
What does this mean for my training in SIM mode?
Fliiiight's grade limitation has no effect on how fast you'll go in Sim mode. Your speed in Zwift and other simulation games depends on your power, and you'll need the same total power output to get up that big hill or to finish your course either way. It does affect how much you'll have to use your gears.
You know of course that Speed x Force = Power. For Fliiiight, Speed also increases Force (more aluminum passing through the magnetic field = more eddy currents). So increasing your speed doesn't just increase power linearly, it squares it. That means you can get very high max power by using your bike's fastest gears.
But you don't use your fast gears when going up a 10% grade, right? Yet it provides lots of resistance, no matter how slowly you go. To represent a very steep slope then, you need more units resistance per units wheel speed, and the maximum Newtons per m/s on Fliiiight is limited by the amount of aluminum in a typical bike wheel.
As for the opposite question - why do other trainers in the same price range have lower max power, despite their higher grade? Usually there's some mechanical component slowing your wheel (gears, chains, belts) that can only handle so much power before it's damaged, so they'll reduce the resistance past a certain speed. Fliiiiight's power-dissipation mechanism (turning it into heat in your wheel, which is then cooled again by air flow) doesn't require any super strong components.
One more factor you might consider, is what does the 7% grade limitation actually mean for your training?
In Zwift, unless you've changed the trainer difficulty setting, it won't have any affect. The default setting is 50% hill strength, and the hill grades almost never exceed 14%.
If you changed hill strength to 100%, you'd still travel through the simulation at the same speed on both trainers, but on the trainer with the higher grade you'd use your gears more. On Fliiiight, you can experience what feels like a steep climb resistance-wise, by not gearing down as much.
The maximum gradient is determined by the amount of resistance that can be applied at slow wheel speeds.