> This whole “study” is based on data gathered through a “viral Facebook quiz” in which people provided self-assestments of their own skills and learning process.

The quiz (which you can take yourself at http://archive.gameswithwords.org/WhichEnglish/) measured syntactic competence with various tasks that were carefully tuned and based on prior peer-reviewed research [1, sec. 2.4]. At no point did quiz respondents provide self-assessments of their linguistic ability, beyond demographic information like identifying their native language and year of first exposure to English.

[1]: https://l3atbc-public.s3.amazonaws.com/pub_pdfs/JK_Hartshorn...