It is, let's recapitulate why it is. Look at this position again:
The question is, is it a chess position?
If the answer is "yes", then the white king can't castle.
Conversely, if the king can castle, say, to the kingside, moving from c1 all the way to g1, with the h rook moving to f1, then it's not chess. This is what I call "retarded castling."
Nobody in the world that has learned chess, that doesn't know about Chess960's retarded castling, would say that the king can castle in that chess position.
If it's not a chess position, and the king can in fact castle like a retard, then it could be any other game, including one where white can drop a pawn on b6, or move the H-Rook somewhere and make an Archbishop appear on h1, or anything you can imagine. We just don't know, because if it's not chess, it could be anything else.
In other words, there's a long list of illegal moves that could be played in the board on the picture I posted, but the list of white chess moves is so short I can list it here:
Legal pawn moves:
a4, a3, b3, e5, f5, g4, g3, h4, h3.
Legal Bishop moves:
(Light) Bxa6, Bb5, Bc4, Be2, Bf1.
(Dark) Bxf6, Bh6, Bh4.
Legal Knight moves:
(Light) Nb6, Nxb4, Nxc7+, Nc3, Nxe7, Ne3, Nxf6.
(Dark) Nb5, Nb3, Nc6, Nxe6, Ne2, Nf5.
Legal Rook moves:
(Light) Rd1, Re1, Rf1, Rg1.
Legal Queen moves:
Qd1, Qe3, Qe2, Qf2, Qf1, Qg4, Qg3, Qh5, Qh3.
Legal King moves:
Kb1, Kd2, Kd1.
See how O-O is missing? O-O-O is also missing. It means those aren't chess moves, so any Chess960 game that leads to this position where O-O and O-O-O are legal isn't chess.
Just like @b6 (pawn drops on b6), Rd1@Ah1 (Rook to d1 dropping an Arbishop on h1) and an innumerable amount of moves are illegal chess moves (so they're not chess), castling and long castling are in the same category. The reason they're allowed on FRC (generalization of castling or otherwise) doesn't make them chess moves.