Late ancient authors, dealing with various ethnic groups not mentioned in earlier historians, often applied to them the names of unrelated people who had lived in the same area many centuries earlier who were mentioned by Herodotus and other early writers. So tribes living north of the Black Sea might be called Scythians even if not really Scythian. It may be that Bede was following that practice (although the original Scyths were never as far west as Britain).
I also had read that calling peoples “Scythian” for anyone from above the Black Sea was common practice in late antiquity. Bede was a learned churchman known as a great scholar and it’s hard to imagine he would use that word while trying to describe Scandinavians. The horse culture of the Picts doesn’t square with Scandinavians anyway.