Aaq, i think the fact that the gemara says that we’re concerned that he directed his bracha to Har Grizim indicates that we’re talking about actual kusim, who worshipped the mountain.
Im not sure it’s a question of shituf or a question of which kusim believed in complete avodah zara. Either way, all evangelicals believe in avodah zara, which is different from kusim, because some clearly would make brachos to Hashem Himself.