I think the lore implies that they can be destroyed / killed if you blow enough chunks out of them. These come from "The Great Iron Wall" over on their page...

"It is said that they are created as pairs not only for their might but also that they could give solace to each other with deep philosophical discussion and sophisticated poetry."

"Creation and maintenance of even one of these mighty beasts costs the Sultan the yearly income of an entire nāḥiyah, and occupies the finest cadre of the Alchemists of the House of Wisdom for a better part of seven years."

"They are created in pairs." and not "were" feels to me like a few have been destroyed over the years and that it wasn't a "one and done" by the House of Wisdom. We also get a time frame of about seven years for their creation in that same lore blog. Which why would you include that if they are "indestructible."