I agree there’s no magic in plywood itself. But there’s still quite a bit that makes one cab different from another, even if both are birch ply.
Internal dimensions, panel thickness, bracing layout (if existing), porting, and how the baffle is mounted all change how the cab sound. Even two 1×12s of the same external size can feel totally different just because one is lightly braced and the other is more rigid. These differences is what makes us go from “boxy” to “open” or “tight” vs. “loose” etc.
So yeah, no magic — but the small design choices stack up and make a bigger difference than people sometimes expect.