It looks like it's designed for the IDE use case - on cursory inspection of the code, it contains an autocomplete database and a client-server architecture designed for editor plugins (with useful interfaces like "type at character index").

I'm surprised that they don't seem to have launched with a public editor plugin and that the documentation doesn't seem to mention it.

The source also has an emacs plugin, named flow-types.el

Here's one :) https://github.com/facebook/vim-flow/

There are ambiguous case where you need to specify a type to obtain the expected error.