My point is that it's definitely not in the "no possible way the PCs could know without cheating" category. It's not really any more esoteric than knowing that Strahd presumably has a Legendary Action to let him move without taking opportunity attacks. (I'm AFB but I think he's just like a normal vampire, plus spells and lair actions but with regular legendaries.) This thing is a bigger, tougher animated armor, that's all. For all we know they cover this in standard Arcana 302: Investiture and Intent.
(On the other hand, there is no wizard so maybe no one in the party has ever studied Arcana.)