That's Brett saying he wouldn't knot Hayden even if he was in him. Hayden doesn't have any dialogue there, it's a flashback to when he was completely out of it during his rut. The thing is Hayden previously couldn't remember that night at all so this is all new info for him. The moment is Hayden subconsciously realizing that Brett does have feelings and does care about him, not just physically, which makes him fall in love and form a bond. He is head over heels for this man.
Brett doesn't know how to express his emotions like a regular person and I think Hayden is realizing that's why Brett acts like he does.
Am I tweaking or what? Hayden tells Brett he wouldn't knot him even if he was inside him, but clearly he's knotted inside Brett on two separate occasions: Chapter 9, when they first had sex, and Chapter 12.