The media decides who becomes a yonko (or pirate king for that matter)
i am not questioning that. what i am saying is that it makes no sense for law and kid to be seen as yonko.
when luffy started being called the fifth emperor we saw that all those factors were counted it was not just the fact that morgan said he defeated big mom but also the five thousand fleet with many pirate crews in it and all the territories loyal to him.
law and kid have the strength but that is not enough to be considered a yonkou. teach was not instantly considered a yonkou when he got shirohige powers but only after he conquered territories and expanded his influence.
if law and kid become yonkou just because they defeated a yonkou it is pure fanservice