Damn, this indeed will need an entry in the Errata/Clarifications.
The intent is that you need more Friendly that enemy pieces to place a stronghold, and while for most of the game that "enemy" equates "barbarian" for both Britons, the other Briton pieces are indeed "enemy" after Fragmentation... So both 3.3.1 and the play aids should say "enemy" rather than "barbarian"...
Sorry for the confusion :-(
... whew, Thanks Marc!