AITA for telling my nephew he can’t call my brother’s friend uncle?

I(21F) have known ‘Gareth’(26) my whole life. He’s best friends with my brother(26) and good friends with me, too. Gareth and I were babysitting my nephew(5) the other day when Gareth gave my nephew a new stuffed animal as an early birthday present. My nephew hugged him and said ‘Thank you, Uncle Gareth!’

Gareth smiled but I told my nephew he shouldn’t call him that since they are not actually related. The only way Gareth would be his uncle is if he married me. I just want my nephew to not use terms incorrectly. That’s all. Gareth frowned and said that that isn’t fair since he’s known my nephew his entire life and they are close.