They were allowed, but clearly discouraged, and that's enough to be in breach of the law. From the article:

> "No differentiation based on sex was made between visitors at entry," the ruling said. However, it added that "public statements which clearly discourage" a certain group from attending an event amounts to discrimination, and therefore the festival was in breach of the law prohibiting gender-based discrimination.

The organizers had made public and explicit statement that "cis-men was not allowed", but then did not enforce it at entry.

Also, a "man-pen"? Just imagine if the targeted demographic was something else, like Muslims.