Question

There, Rabbi Yannai derives from “you also” that a gentile cannot be an agent. And the Talmud asks that for this no verse is needed, in accordance with the view of Rabbi Yohanan that since a gentile is not subject to that matter, he also cannot serve as an agent.
And this is difficult for me: what Rabbi Yannai derived is not that a gentile is not subject to that matter, but that even if he were subject to that matter, he still would not be able to serve as an agent.So why does the Talmud challenge Rabbi Yannai on the basis of Rabbi Yohanan?