Islamic coins would make sense as Western Europe had collapsed and the Vikings made a log of money selling slaves to the Islamic world.
You are probably right about slaves, but I introduced an error on a important detail — the article actually says “Arab silver dirhams”, and I conflated that into “Islamic”. The former might be vastly older than the later. Two hundred years for the coins to travel from Byzantium, then through Ukraine & Russia to Scandinavia, then on Viking voyages to 870’s England seemed fast, but “Arab coins” expand the timeframe immensely. It would be interesting to hear a coin expert’s view of the speed that coins would travel in those times and places.