Hi. On this study, if I go to puzzle 10 (that is, chapter 11), play through the solution, go the analysis board, and back up a few moves, it clearly shows castling with the rook on a8 and the king on d8 as a legal move for black. You can play the impossible castling move on the board, and it also shows up in stockfish analysis lines. Seems iffy.
In the board editor you can choose if castling in the given position should be possible
If you choose to have castling on in a position where is should not be legal Fisher random castling is used