Wow. Them Roman traders really got around!
I think that fermented fish may be a very ancient part of the human diet. To me, it seems like and offshoot of methods used for preserving meat (especially salted meat). My guess is that some ancient human decided that if salting was good for meat, then it MUST be good for fish. Then, the idea spread from there.
Pompeii proves Romans had oranges from southern China. Its all about the money the traders could make. The ancients were far more clever than we think. What about those Egyptian mummies with cocaine?