The reason we have always been told not to refreeze - frozen foods from most grocery stores come in trucks that have traveled all over the place stopping, opening the doors, unloading, handled by who knows how many people and during the process it partially thaws some or maybe even completely. You have no idea how many times this has happened. By the time you purchase it and get it home it may have been completely or partially thawed 4 or 5 times and possibly more. All fish and seafood thaws rather quickly and has probably been thawed or partially thawed several times before you get it.
So there you have it. If its food you processed and you know you did it right the first time, you can safely refreeze anything including fish.