well possibly and i definitely thought he wouldve been immune to taking damage unless they hadnt said the thing about needling him with small attacks to throw him off balance. so maybe he techincally is unkillable by attacks but hes not immune to damage (kinda like a master hand without a breaking point)isn't that what invincible has always been known as? every game where you're invincible you can't take damage.
and really don't rely on the translation on giga bowser. "He must be invincible" is probably a translation error. maybe in japanese it says "He IS invincible"