"Created graphic assets / brand guidelines to help subcontracted artists maintain cohesive product identity."

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Can someone help make sense of this? I don't see how you can create "brand guidelines" for "subcontracted artists" but also be "completely uninvolved" with the process of contracting the game art.
A graphic designer that creates brand guidelines doesn't necessarily choose who a firm contracts with or talk to them directly. Those are separate roles, and it seems Vito was pretty junior at that time anyway, which makes it even less likely he was client-facing.
He may have written a creative brief for each composition, but that doesn't mean he's directly involved in communicating with the agency that makes the art.
Brand guidelines consist of color palette, typography, and various other design elements (including artistic and stylistic references in this context).
"Art" and "graphic assets" are not interchangeable terms. Think of it in layers. The art is the underlying visual (literal art) and the graphic assets are the overlay that consists of typography, icons, and the various visual containers and information that convey information on what that card does.
A music album is very similar. You have an artist or photographer who creates the art and a graphic designer who does the layouts (logo, typography, actual layout such as song listing, lyrics, credits, etc).
Obviously the studio was small at that time, so maybe he did communicate with them. Even if he did talk to and contract them, it doesn't really matter.
I assume the agency that created the art either didn't request credit (most creative agencies offer their services on a white label basis but reserve the right to credit themselves in their own marketing materials) or the lack of credit was part of the contract. It's not a "mean" or underhanded thing to do and very common in the industry generally. I have no experience working in this industry in particular, but none of this sounds abnormal.