This study focused on grammar. They didn't test for fluency, which is generally defined as the ease of using a specific skill like speaking, writing, reading, or listening, but is sometimes used in a broader sense (usually outside of applied linguistics) to mean overall proficiency.
They tested one aspect of knowing a language.
Here is a link to the journal article: http://l3atbc-public.s3.amazonaws.com/pub_pdfs/JK_Hartshorne_JB_Tenenbaum_S_Pinker_2018.pdf
And here is a link to the test they administered: http://archive.gameswithwords.org/WhichEnglish/