I just about flipped a table when I glanced at that image and then started reading about a supposedly 13th century map. I find the use of that image at the top suspicious, perhaps akin to link-bait.
I'm with you - it's also extraordinarily obvious given the level of detail/explored regions represented - relatively surprising that the author did this given the level of detail the article goes into...but may not have been her call though
This research doesn't seem to be published anywhere, but here's a 30-page presentation at Hessler's academia.edu page, which contains some nice data plots.
I have found one editorial by Mr. Hessler, one review of his book, one review made by himself and a paper (Warping Waldseemuller: A Phenomenological and Computational Study of the 1507 World Map, printed in cartographica) but is is about a different piece of cartography.
That region was barely explored at the time -- the coastline in that area was probably based on observations from only a few voyages. (Given the harsh conditions up there, it's also likely that the observations were subject to more error than usual.) I wouldn't put too much stock into it.
As others have pointed out, not much was known of that area at that time (1746).
In James Boswell's "THE JOURNAL OF A TOUR TO THE HEBRIDES
WITH SAMUEL JOHNSON", which occurred in 1773, he writes:
"Talking of Phipps's voyage to the North Pole, Dr Johnson observed, that it 'was conjectured that our former navigators have kept too near land, and so have found the sea frozen far north, because the land hinders the free motion of the tide; but, in the wide ocean, where the waves tumble at their full convenience, it is imagined that the frost does not take effect'."
So there was certainly speculation about the Arctic at this time by educated people, but clearly they didn't know a great deal about it.
Yeah the facts that 1) the arctic ocean is covered in ice even in open water and 2) The north pole is in the arctic ocean (rather than on some potentially undiscovered land) weren't conclusively proven until the Fram expedition by Fridtjof Nansen in 1896.
There was enormous speculation in the papers and in the scientific communities about what would come of Nansen and what he would find in the lead up to during his expedition so practically speaking the extremely high northern latitudes remained a "blank spot" until the turn of the 20th century.
