Didn't think it was possible to drag me out of my vacation, but here I am I guess. I'm just gonna hash this out real quick, starting with the easy one.
1. Ignoring Honkai 3rd feats is stupid.
Whoever asked to delete the Honkai 3rd profiles is either hopelessly clueless or actively malicious. Those profiles were perfectly fine (I should know, I made practically all of them personally) and there were people working on revising the verse. Whoever is then claiming that Honkai 3rd is suddenly invalid purely because those pages are no longer around, despite it being explicit where Welt came from, is even worse. The removal of a page is not the same as the removal of a feat. For as long as the source calc on the speed Welt scales to is considered valid, it cannot be removed from his profile.
And now to the main event...
2. A piercing attack affecting something of a higher tier only contributes to a higher rating.
Which is to say, it does NOT provide enough evidence to be a standalone proof of a higher rating. Unless Argenti displayed similar feats of clearly and notably damaging a 4-A character, then piercing the Giant Sting's stomach is an outlier and nothing more. The fights against Emanators don't count for much, because as I've brought up before, there are no 4-A Emanator feats. The two biggest feats in HSR right now are Zephyro destroying a star system and Dominicus trapping all of Asdana in a dream, neither of which give 4-A for reasons I assume everyone is already aware of.
Just because sharp objects are not considered durability negation, from an objective physics standpoint, a human-sized spear piercing the stomach of a monster large enough to hold a starry sky in said stomach is the equivalent of a needle piercing through a sandcastle on planet Earth. If anything, it can be argued that the spear would realistically do less relative damage; if it weren't for said attack causing spasms in the monster's body. The only reason VSBW doesn't calc force over surface area in the context of blades and needles is - to greatly oversimplify - because it's a hassle to keep track of and inconsistently portrayed across media. Yet even so, there's a reason environmental destruction is not considered in the same vein as raw kinetic energy: just because a storm totals to a certain amount of energy doesn't mean the part of the storm actually hitting its targets has all of that energy being applied directly. This applies both ways.
This isn't even getting into how "direct attacks on internal organs" are considered a valid form of durability negation. Dozens of characters have abilities that control blood or manifest effects inside their targets and those are considered durability negation even when said characters use them on opponents relative to their normal tier. Argenti piercing through the Giant Sting's stomach would be considered in the same vein even if he was already 4-A, which he currently isn't.
So no, Argenti does not scale to 4-A nor does anyone else.