Are you sure? That seems like a very low limit for such a large bag. The touring bag is 31 liters. On
this FAQ page, Brompton says the bike is designed for rider + 20 kg of luggage (presumably front + back).
No I am not sure where I got that number from - I can't find anything official to suport that number. The two bolts that hold the mount to the Brompton frame are small (m5?) and the plastic coupling itself must be under a lot of strain at high weights. I may well be wrong, but ~5kg actually seems good on both Brompton and Birdy.
I didn't mention it but I upsized the bolts to M6 on my adaptor. I have also tested the Birdy+ Brompton bag with 10kg in it and it didn't break off, but I didn't like the bike's feel much.