This is frowned upon among theater people: writing about a new play before it officially celebrates its premiere. Something could go wrong at one of these "previews". The staging could still change. Or a main actor falls ill and has to be replaced at short notice. On the one hand.
On the other hand, the organizers are busy selling tickets, which are at least discounted, to an audience that can't wait any longer or doesn't want to see a spectacle that is set to become a cultural hotspot for Cologne residents and visitors to Cologne in the next few years.