I think OPs point is, there are other perfectly coherent grammars that also may produce the same result. Were those grammars tested?
And a good follow on would be, if not, why was only one and how/why was it selected?
And a good follow on would be, if not, why was only one and how/why was it selected?
Just my first thought about the article is that it is doing this "think of the children" type of thing to make a case for recalling something that is really useful for many people.
So I would imagine that any deviation of this expected norm would yield in a result that might be perceived as more "stupid or lazy", whether it's street dialect, US southern, redneck dialect, teenager dialect, mix with other languages or otherwise.
So my point is - that it's measuring a correlation from the standard, professional, business English, rather than it being anything to do with racism.