You know given that the Steel sidebar likely uses the Silver's general statics like in Mad Mage, wouldn't it be easy to convert a Steel Greatwyrm as well?
Doubly so if Song ends up using Copper like people are theorizing.
Best thing about DnD is that you can pick and choose your own lore.
Even then given he isn't dead, it's completely plausible to have one of thoses 'fragments' be a Greatwyrm Gem that just has the abilities of every type and acting out his will. That and maybe a few other things would be what I'd...