We accept that Res Ulq's Cero Oscuras vaporizes, we also accept that anyone stronger than Res Ulq's Cero Oscuras vaporizes, as indicated by Espada 3, 2, 1, and 0 all scaling above Res Ulq's Cero Oscuras on their profiles.
Gran Rey Cero is the ultimate Cero, therefore Res Ulq's most powerful Cero would be if he used a Gran Rey Cero. Res Ulq's Cero < Res Ulq's Cero Oscuras (vaporizes) < Res Ulq's Gran Rey Cero. Res Ulq's Gran Rey Cero would vaporize by proxy of being a more potent Cero than Res Ulq's Cero Oscuras, which already vaporizes.
Gran Rey Cero can destroy Las Noches, and as you can see with the translation and scan provided by IMade, Res Ulq's Cero Oscuras can also destroy Las Noches. Additionally, we see that by merely grazing Las Noches Res Ulq's Cero Oscuras vaporized a fourth of it, the statement
highly implies that had Ulq actually aimed his Res CO at LN he would've vaporized the entire structure, and his GRC is even more potent and can destroy LN.
The opposition is saying the GRC feat is merely too vague to assume the type of destruction. However, for characters that scale to or above Res Ulq, that is simply not true. We Res Ulq accidentally vaporize a fourth of the castle via grazing it with a weaker Cero, it's implied with statements that had Res Ulq aimed CO at LN it would've made it go bye bye, and then GRC is more potent and has another statement for being able to destroy LN.
It's obvious that for characters who scale to or above Res Ulq, their CO's can vaporize massive chunks (if not all) of LN, and their GRC's (most potent Ceros) should be able to easily vaporize LN.
That's the low down hoe down.