"It’s just a skin” really sums up that you don’t know about how a video game might be developed.
Leaving aside the pessimistic nature of OP for a moment, by definition Reforged IS just a reskin, in the end. It should be, otherwise why are they even calling it “Reforged”.
Let’s see what the definition of the verb “to forge” is.
To forge: “to form by heating and hammering; beat into shape.” (Source: dictionary dot com)
To reforge means to reform something, and give something another shape. So you do start with an initial shape and you end up with another shape of the same thing.
They may “reforge” the soundeffects, spell effects and so on but it still falls under the category of a “reskin”.