Ayanokouji himself was the red herring in the previous arc.

Horikita seems like she has very little experience with people superior than her, so she probably, like a child, did not understand how it felt like to be the weaker person in a group. Ayanokouji basically showed her her place.

Why is Ayanokouji's grip strength so strong, though? For what purpose would he train it while intentionally keeping himself slim?