What propels you forward is force not torque. A small wheel requires little torque to generate a given force, whereas a large wheel requires more torque to generate a given force.
Thus, on a small wheel like a Brompton you would use a big chainring and a little sprocket in the rear to create a low torque at high speed. On a 27.5" fatbike you would use a small chainring and a big sprocket in the rear to create a high torque at low speed. The most problematic for the gear hub is the small chainring and big sprocket on the large wheeled bicycle.
What you as a cyclist see is force. What the hub sees is torque. The hub can only withstand a certain amount of torque. For example, for Rohloff they assume the torque at hub never exceeds 130 Nm. However, with 1:2 sprocket to chainring ratio a 110kg cyclist using clipless pedals (thus pulling up from the rear pedal at 25 kg) and pulling up on the handlebars at 20 kg when sprinting would easily create 300 Nm at the 170mm cranks and thus 150 Nm at the hub.
The most hard time a hub is having is on a large wheel bicycle. A small wheel bicycle typically puts smaller torque on the hub due to the different sprocket to chainring size ratio. Bicycle wheel sizes go up to about 27.5" fatbike, you won't see larger wheels anywhere else except perhaps penny farthings that don't use geared hubs. 27.5" fatbike is not that much different from for example 28" (622mm bead seat) city bike or road bike. However, children's bikes and specialty folding bikes use very small wheels.
Also note that the torque at the hub is only dependent on rider weight, rider strength, riding style, crank length and sprocket to chainring ratio. Wheel size does not matter. So for example with the same 1:2 sprocket to chainring ratio the strong sprinter could sprint on a Brompton for example. It would then create too 150 Nm at the hub if the sprinter can use the Brompton handlebars as efficiently as drop bars. The gearing would be very low so to provide any "resistance" to pedaling the 150 Nm could most reasonably be achieved when sprinting on a steep uphill. On a flatland, achieving the 150 Nm at the hub might not be possible because the bicycle would accelerate so hard below the cyclist that the cyclist can't maintain the maximum force on the pedals.
Oh, and also after reading the question a second time more carefully: yes, you can use 8.5 gear inches on Brompton. You can use anything larger than that. You cannot use anything below 8.5 gear inches. I think it's self-evident that you won't be gearing it to lower than 8.5 gear inches.
The lower the gear, the more torque the hub sees. Thus, it's not high gears that the rule is trying to prevent you from using. It's low gears.