Thanks for the correction.
I remember looking into this before and finding I had lower capacity, but looking again, I see that is with the ZiZZO "pignose" adapter, which has a 15 lb. limit and is compatible with Brompton:
https://zizzo.bike/products/zizzo-he...e-adapter-30mm

Apparently, the genuine Brompton one has a higher limit, but one has to also consider the capacity of the adapter that would go between the Brompton luggage block and the brazed-on mount on the head tube.

We can see that Tern's Luggage Truss cantilevers out further, which amplifies the stresses by adding leverage. Looking at the setup on my bike, we can see that the reason is to allow room for the brake & shift cables to move around without interfering with steering. With their Luggage Truss, Tern supplies a velcro strap for the cables to hold them out of the way. If the Brompton block + adapter has the bag closer to the steering tube, cable management can be an issue. If there's not room for the cables behind the luggage and it's tight to route them off to the side, they cables can bind while steering, causing a crash. It's just an example of how the OEM solution was well thought-out, while the DIY solution may not be.