Technically you are correct, surrealist art by definition should be devoid of constraints. However this was not a surrealist art competition, its a “surrealist art in the style of JACEK YERKA” competition. The constraint is inherent in the brief. If people feel that the brief is not in the spirit of the surrealist art genre, then they won’t enter the competition. If people think its exploitative or restrictive, then they won’t enter the competition.