That scan has a lot of iffiness about it. It's Kang calling an enchantment "simple", not the same thing as Thor or even RKT finding it simple.
The _safeguard_ isn't the Odin's blood thing (which just seems to mean, any descendant of Odin), but the weapon itself. Thus reinforcing the idea that it can seriously harm The Celestials.
Still, all this weapon does is pierce their "armour", which really isn't saying that much.
We know they exist beyond simple physical form and that they can reconstitute on a whim, so why would this weapon hand any the fight to RKT, even if he were allowed to summon it.
Also, The Celestials, like Galactus, always vary the strength of their attacks depending upon the seriousness of the threat facing them.
Galactus never one-shot killed Hulk, Surfer or any herald, yet he one-shot wiped out 3 star systems and annihilated a Watcher as a by-product, when he got serious.
The Mad Celestials, individually were not as powerful as Galactus, but in combination they clearly surpassed a well fed Galactus.