YTA. You treated him like an animal, not a person. Animals have no responsibility and so the animal’s keeper takes responsibility. You apologizing for your BIL’s behavior implies he has no moral agency. It’s dehumanizing.

Also, you assumed his feedback was ill-intentioned. Did it cross your mind that he was giving them advice on how to increase their sales? That it’s legitimate for him to express when he dislikes things? Did he use a snotty tone when he said this? Did you apologize on the girls’ behalf for annoying him with a weird sing-song pitch?

Yeah I’d be mad too if you did that to me. I can’t even stand it when a parent apologizes for their kid.

A person holding back an unruly dog says “sorry” because they haven’t trained the dog well though yet. A person with a human acting in a way they don’t like takes it up with that human. They don’t apologize on that other human’s behalf.