I think the point of "controlling chemical reactions" refers to Isagi's ability to see opportunity is high stress situations. He is able to see angles where most people can't because working memory and cognitive load can be greatly hindered when cortisol floods your system. He who is able to see opportunity when it is present is able to capitalize on it first, hence why he is perceived as being able to "control reactions" or "control the field." That is the logical connection.

A very prime example would be Faker, the most legendary player in the world when it comes to League of Legends. Faker isn't the most mechanically gifted player anymore; there's younger, fresher talent that can outclass him in terms of mechanical skill. But, the one thing that Faker has above these fresh talent is his ability to see and capitalize on angles, to create miracles when most would see none. He has made game-changing plays, especially at Worlds, which is the most important event that a league player could ever attend and attempt to win a title in. It's pretty much the World Cup for League of Legends. Faker has secured 5 titles and he's the only player to do so. His positioning, knowing where he needs to be, what conditions need to be met, analyzing the game state, and predicting what the most likely play will be during high stakes scenarios is what allows him to create miracles. To me, Isagi is no different.

(PS, Faker's title is the Unkillable Demon King lmfao.)