Arabic is both a (group of) language(s) and an alphabet/script. The page isn't claiming that 2B people speak some dialect of the Arabic language, it is claiming some 2B people have a degree of literacy in the alphabet/script, so a quarter of the world's population will potentially know if you've screwed up your website/app/game/whatever. You don't need to speak Latin to know that "anunmtMh mucjmydDDDDD" is just the result of me mashing random keys on my keyboard, because you can read Latin letters. Likewise no Mongolian speaker would believe "прнтзх йоэЁюи" is proper Russian, even though those two languages are wildly different.