The whole code browsing experience is just different, you can actually read partial sections of the code base and understand them, use code completion, "go to definition" and "find all usages" and it works reliably. Not using types is in my book professional misconduct.
His comparison with C#/Java is also very weird and shows he hasn't given it a proper try. The ducktyping nature of interfaces makes the code much less rigid than in java but still maintains most of the safety you need.
TypeScript definitely got the structural checking of interfaces right, but I beleive (correct me if I'm wrong, I'm referring to https://github.com/Microsoft/TypeScript/issues/202) that classes are also structurally checked. Flow does nominal checking for those, with very good reasons.
While it's a shame the details are light because you kind of need the presenter to go with the slides, the following explains a few of the reasons between TypeScript vs Flow:
http://www.scs.stanford.edu/16wi-cs240h/slides/fb-slides.htm...
(not the whole deck, just a few slides after 45)
Typescript doesn't have real algebraic data types becauses at the end of the day, Typescript is just a statically-typed version of Javascript. And Javascript doesn't have adts.
For me, the biggest advantage of static typing is that code is much more self-documenting. Also, refactoring becomes much easier.