In other words, your claim that “there is an opinion that coffee (made from coffee beans) is kitniyos” is false.
Just as it is false that there exists an opinion that potatoes are kitniyos. There is no such opinion, no rov has ever forbidden potatoes, and anyone who claims otherwise should cite a primary source. Secondary sources do not count. And if you cite the Nishmas Odom you automatically lose, because it will prove that you’re citing a secondary source and didn’t bother to look it up yourself.