> philosophically speaking, having hope is a rather subjunctive matter, isn't it?
Yes, you could say that (and you did). You need forget about philosophy, and psychology. You're quoting from a chess primer.
The author, note carefully, is talking about black, ie. the pieces, not the human opponent.
More specifically, he is talking about a configuration on the board.
By saying 'black's cause is completely hopeless' he is merely injecting a little colour into his writing with a hint of metaphor and a dash of anthropomorphism.
The chessmen, and ultimately the configuration of those chessmen, are sublimely indifferent to any abstract value we may happen to assign to them.
'black's cause is completely hopeless' = with best play, the game will conclude in one of several configurations known as checkmate for white (it could get a little wearing, don't you think?).