The tech sheet says ALL penetrations need either one of their seals, or KerdiFix to ensure there is no chance of moisture getting inside of the board. Normal joints/seams require the same thing as Kerdi sheet membrane - a minimum of 2" overlap of material with an unmodified thinset bonding. The inside of the shower/steam room, when using KerdiBoard must be made waterproof and penetrations properly sealed to prevent moisture or vapor from intruding into the system. As I said, I have neither touched nor seen KerdiBoard, and other than the tech sheets, I don't know any more about it.
For others that may read this, the shower in question is a commercial steam room, likely run many hours a day. From what I read, Schluter is on site trying to help understand what went wrong and how to fix it as any of the quality companies out there typically do when there's an issue if you contact them.
Let's not come to a conclusion until more is known.
A typical home steam shower is used probably less than an hour a day, if that, giving the whole thing a chance to dry out if vapor did get places you'd rather not get it. Schluter does not approve of gypsum based backers for commercial steam showers (per their Kerdi shower handbook installation instructions) that are likely to be used much more than that. No membrane is 100% given high steam concentrations, and an inert backer is required when it is likely it will have minimal chance to dry after long-term, continuous high vapor pressure exposures of a commercial operation. For a home, a typical steam shower use probably isn't worse than a typical teenager's long, use all the hot water shower, and for those applications, accepts the use of numerous backer materials since there's typically a large percentage of the day when it is unused and drying.
One thing to consider, steam can get VERY hot, just like any gas. While there's a limit on how hot liquid water can get unless under pressure, there's no such limitation once it becomes a gas. There's no way to know what temp the steam was in that system that could have been run 24/7, and the steam line ran, uninsulated for the most part, right behind the board. A typical home steam shower, the pipe comes straight into the shower, and doesn't run long distances against the backer , entering at a 90-degree angle with minimal contact with the backer, keeping the run short to maximize the effect and efficiency of the system. A commercial system could be feeding multiple shower rooms, and would likely run hotter and would need to be piped further.