From the rules page:
Mercenaries surround the king from all four sides and capture him. The second possibility is that mercenaries surround the king from three sides and the throne stands on the fourth side (since nobody, including the king, cannot move to the throne). The black player wins the game.

If the king moving into that dosn't mean a black win, then the rules need to reflect that because as the rules are written now, black should have won.