You're right, this has been added recently and there's also now a community effort around 3rd party modules. [1]
Though it looks like it didn't become active until after at least 03/2016 [2].
I mention this only because TypeScript has been active in this area since 10/2012 [3], which is in-line with the original intent of my comment - that Flow is over 2 years (closer to 3.5 years) behind in building this out.