I borrowed the book at the library.
Basically his argument is:
- Tolkien had intimate knowledge of Jutish history from his scholarly work (Finn & Hengest) and actively used this in his writing (Cottage of Lost Play).
- Tolkien really liked maps.
Therefore Tolkien had to be aware of the place names Aros, Hjelms Dyb, Eskerod and Isengård when writing his tales.
To get to this very compelling argument one has to read through 170 pages of the author (badly) retelling the pre-Danish conquest history of Jutland, and outlining Tolkien's work on the Finnsburg fragment.
As an extra slap in the face, he introduces his argument by admitting that he has absolutely no proof of his theory...