I feed Swiss and rainbow chard to my lot which is safe and they love it! I cant inagine one certain type being poisonous. It can be a food with a fair bit of calcium though and gets labelled as 'gassy' from time to time. Youll often find some sites will just class somethig as 'toxic' or poisonous without any real reason why, especially if its a food they should only get in moderation due to confusion or lack of understanding.
From what I've read, all types of chard are safe and I'd be inclined to believe that as it seems logical to me that it'd be all types or no types at all would be safe.