It would be interesting to know if footprints of past conquests/empires are the reason for the line not correlating with today's borders. At least the two pockets on the south Iberian Peninsula and as well the southern Apennine Peninsula correlate with former Islamic Caliphates.

I wonder though why Ireland, Austria and Venice would be outside. In some respects all of them played central roles in European history (Ireland in Christianization).

EDIT: Just figured out, the drawing is off. If drawn correctly it would include Vienna and Venice.