The big question there is why these metalworkers showed up in only one part of the empire, one of the less wealthy regions, and not one with some unique needs for bronze casting. Note that these are cast bronze, using the term "metalwork" conflates smiths who work with tin, gold, bronze, and iron, but those were different professions. This isn't a blacksmith showing off his ironworking skills, it's a person of a different profession casting fine bronze. Hard to say why you'd have mendicant bronze workers casting things, I don't know of any records of but it's a very limited area for so many finds, while this same work was needed all over the empire, why nothing in Spain, Italy, North Africa, or the eastern provinces where they'd need the same? Why just that one highly restricted range within Rome that coincides with Gallia and Britain?