Grammar and speech does in fact belong to the individual, because grammer is simply a description on how people talk. That was how it started, the scolars way back started to gather information on how people talked, and compiled this information into a 'grammar'. Nobody made it up, and made everybody speak like that.
A language is defined among other things as something a number of people agree on and can use among themselves to understand each other. If that were not the case, no one could talk with each other. Neither the article nor I suggest that each person suddenly decide to invent their own - it would not be a language.
What I say is that there is no real 'tick-list' for 'correct' language, only a snap-shot on how it is talked now, and that is changing. ('Snap-shot' taken with a grain of salt, as language takes time to change.)
Grammar was not a sort of divine intervention, given to mankind, and to be obeyed to the letter - sorry, could not resist that ;-).
It is made of how we all talk, and no board or scholar owns it.