It's not very comprehensive at all, the regions I know well in France are missing almost all of the actual castles. The site lists 180ish castles in France, when it's estimated there are 45,000 castles + chateaus in reality. Of course this depends on your exact definition of castle.

I wonder if it's only scraping english language Wikipedia articles?

I suspect the Mongols destroyed a lot of Asian castles and city fortifications. They never made it to Japan.