The book can be purchased on Amazon.
Throughout the book, dates are reckoned relative to the sinking of Aldland. It would be strange for the Fryas to reckon time relative to the demise of the land of a foreign people.
In 7b, the sinking of Aldland is described. It would be strange to devote so much wistful prose to the sinking of a foreign people's land, especially one hated by the Fryas. Aldland and “the lands of Finda” are mentioned separately. The R1a haplogroup — comprising many of the Findas — seems to originate from the Altai Mountains, which is not an island that can sink.
In 8c, Inka, who had had enough of the Findas, ventured out into the Atlantic Ocean in search of Aldland, where the old island of Frisland would have been located. Surely he would not attempt such a mission if he had the slightest suspicion that some Findas might live there.
In 3c, Aldland is referred to as the homeland of the Findas, lying under the sea and being far away. Lying under the sea does not fit with their origin being the Asian steppes, so I think submersion was an assumption, since it was described as far away and thus not something the Fryas could confirm. The Fryas had seen many lands sink and rise, forcing their exodus from their own Aldland, so they figured that this was also the reason that waves of Findas invaded Fryasland.