Rather than edit directly, I thought I'd state my opinion first.
I would think that bubble is used in the same context as 'living in a bubble', which is to say isolated from the outside world.
A true bubble kingdom in Erfworld terms would mean
- No interaction with other sides.
- Being self-sustaining. ie. entirely off the Schmuckers generated by their own city\
- Able to remain remain hidden from other sides.
FAQ failed the first two by being dependant on mercenary income.