The numbers are actually correct.
Its my map's graphs that are actually less than clear in terms of colour. The large percentage that looks like the Hesperides/Americas is actually Poland, shown for scale as compared to the rest of the world.
One of the concepts I wanted to show on this map was a world without the holocaust. This comes with a double edged sword of increased anti-Semitism.
And you are actually correct. Poland's population is actually about a quarter Jewish. This high percentage is a product of a number of factors related to this world - but the short answer is immigration of Jews into Poland and the emigration of catholics out of the country. These include:
- Poland having had a more successful history, not having been partitioned. The Jagiellonian dynasty lasted longer, and for a time even succeeded in subduing large parts of Russia.
- Russia eventually threw the Poles out, and eventual dynastic troubles caused a pseudo-French revolution in Poland. This in turn led to an early emancipation of the Jews within Poland and their early incorporation into Polish secular civil society.
- Poland thus continued to maintain its reputation as Paradisus Judaeorum and thus attract immigrants from other parts of Europe.
- Polish anti-clericalism also encouraged many catholic Poles and greek catholic Ruthenians to emigrate to the Hesperides (Americas).
While 15 million is high (it is about OTL's Jewish population today post-Holocaust), I believe it could be possible. Prior to World War 2 OTL, the second Polish republic had about 3 million Jews, while the Soviet Union had 2.5. Additionally, the Americas had about 5 million Jews, most of which I believe emigrated from Eastern Europe and Russia in particular. Since most of Russia's Jews originated in the pale of settlement, which in turn was essentially Russia's part of the Polish partition. Thus, absent push and pull factors drawing people to the America's, it looked like around 8 million Jews had their origins in the old Polish Lithuanian commonweath.
Thus, in a world where Poland did better, and maintained and grew its Jewish population (while avoiding OTL's demographic disasters of OTL's 20th century), I think 15 million could be plausible. I do copt to it being a large number though.
The Hesperides is primarily Roman Catholic, and hasn't actively encouraged Jewish settlement (with the exception of what we would consider the Canadian prairie provinces). The Jewish population in the Hesperides is primarily of Portuguese and Spanish Sephardi descent.
As to western Europe, that is the newest variation of the Holy Roman Empire, built from the foundation of an Anglo-French union (the primary important part here is that Henry V's victory eliminated French Salic inheritance to the throne). It is an (un)holy mix of European union, military alliance under a shared catholic dynasty.