It’s possible Taika Waititi was there in his capacity as a director, as we know that he will be helming one of the episodes of the new series, but the other guys don’t have any official position within the show, as far we know, anyway.
Maybe they’re all being given cameos as Star Wars aliens, that sort of thing has certainly happened before.
It seems that a few friends decided to go by the set of The Mandalorian, the upcoming series that will debut on the Disney+ streaming service that will launch next year, in order to support their fellow director.