Technically the JoJo’s Bizarre Adventure OVA, which covered only the Stardust Crusaders arc, had already existed before GX started, so they could have pulled it from there. Though the fighting video game based solely on JoJo’s Part 3 existed before then as well which is where the “Za Warudo!” meme actually originated. Dio’s Japanese VA was likely picked for the role because of his role as Saiou.