true, both were kidding, of course Grohl knew it was a great performance and was mocking with his comment. They were brothers ... it's normal to banter
i cant be 100% sure but i tend to believe everything kurt said or did was serious under the circumstance that the whole world is just a bad joke. people like him tend to have strong feelings in every direction. so he will have hated everyone, just to love them the next day. in other words... it is very likeley that he suffered from bpd.
I remember one time Kurt did this thing and it was like so funny and then this guy was like wow haha that’s so rad how u did that’ a lot of people were like far out dude but that didn’t matter because Kurt had already said that it was rad’ then after that some guy was back stage and he was like dude and Dave was like yeah dude I know! Then Krist walked in and was like that’s so crazy! I was thinking the same thing! We should go and buy some milk and pour it all over our legs and call ourselves Sally and cut each other finger nails’ it will be such a jolly old time oh what fun we would have 😮