https://www.ef.com/wwen/epi/about-epi/faq/
> What is the methodology behind index?
> The EF EPI calculates a country’s/region's average adult English skill level using data from three different versions of the EF SET. Two versions are open to any Internet user for free. The third is an online placement test used by EF during the enrollment process for English courses.
> In order to calculate a country’s EF EPI score, each test score was normalized to obtain the percentage of correct answers for that test. All the scores for a country/region were then averaged across the three tests, giving equal weight to each test. Regional and global averages were weighted by the populations of each country/region within each region.
People who don't know english in the first place won't have any reason for taking those tests.