The blue there shows the conditions that the trainer can accurately simulate. As you already noticed, a 75kg rider will never get proper simulation of the claimed 7% incline, the best he can hope for is ~4% at low power/speed and in fact at anything above ~400 watts, the best he can't expect any incline simulation, the brake is just not strong enough.



A 50kg rider would get the claimed 7% incline simulation at low speed/power, but not much more than that.

You are right that if you want more realistic simulation, you will need a better trainer. Lets look at the Genius:



Our 75kg rider would get accurate simulation up to 17% regardless of speed, as long as he keeps it under 400 watts. If he wanted realistic resistance for a 17% slope at a higher power (say, 600W) he would need to go faster.

The brake on these trainers can only take so much torque. In general, the slower you are riding and the heavier you are, the more torque you are producing (for a given power) which is why those plots look the way they do. Hope that makes some sense.