Although the all encompassing term refers to the various groups of Germanic people who settled in Great Britain, Saxons actually settled in more southern parts of England.
Alternatively, the Angles established the kingdoms of Northumbria, East Anglia, and Mercia in what we now know as the North and Midland areas of the country.
As Athelstan is established to have arrived from Northumbria, Ragnar and the rest of the Vikings were actually wrong to refer to the monk as a Saxon.
One recent article in Screenrant highlighted their mistake and explained more.
source: express.co.uk