Flowtype isn't sound either. I don't know where this misapprehension comes from or why people keep repeating it; if you simply try to induce an unsoundness in F

Flowtype isn't sound either. I don't know where this misapprehension comes from or why people keep repeating it; if you simply try to induce an unsoundness in Flow you'll succeed after about 30 seconds of trying. An actual sound type system built on top of regular JS would be unusable - you'd have to disallow almost everything. The language is too "dynamic" to become properly sound.