TypeScript supports ES7 features, for example. It also has a few language-local quirks (like parameter properties, described https://www.typescriptlang.org/docs/handbook/classes.html ).

I find the grandparent argument pretty unconvincing though. People writing Flow are generally using JSX or ES modules (the Flow docs recommend the latter, for example), both of which require a transpiler like Babel to be used in a browser anyway.