I think Cooper was only being half serious. He even said (including him) in the post, meaning he recognizes that Cooper is not really on Jakob's radar as a rival. Jakob's only real rivals are people like Tim and maybe a couple of the top 5k guys in the world like Barega.
And then I think Jakob didn't realize it was a half joke and came back with some shots fired, but maybe he was tongue in cheek there as well.