Surprised they didn’t opt for Typescript. Then again, it’s a tweet - looking forward to a longer blog post with technical details when they’re willing to talk about it more.
I assume that they have. Typescript compiles down to Javascript so the tweet is technically correct (written in TS, executed in JS) and there are many out there who won't have a clue what TS is but have at least some grasp of what JS is so JS was probably stated instead of TS to avoid confusion amongst the unwashed masses.
Yes, in theory you _can_ write big projects well in dynamically typed languages, but in practice you get a mess of undocumented interfaces and unit tests that don't quite catch the plethora of errors not present when a static type system is in place.
I think Typescript is a great middle ground between the loose running JS hipsters and the Haskell loving Hindley-Milner type system CS researchers. You can enforce the type restraints that you want (I personally am strict about explicit "string | null" declarations) but you're still hip enough to draw the JS talent.