Sam: the Beis Yosef merely quotes the Rashba without adding anything of his own. The Rashba equates an intentionally added ingredient with Cheese, which is a Ma’amid. It doesn’t seem from his words to be an original category of “not Batel”.
I imagine his logic is that he defines the rationale of a Ma’amid slightly different than you did. I think he interprets the definition of Ma’amid as something that has been designated as significant to the final product. In cheese, for example, the coagulating effect of the rennet makes it significant. In the Rashba’s case, the intent by itself lends Chashivus to the ingredient. Also, presumably if it is an important component, it lends some influence to the final product.