I'm intrigued by this style in javascript but it seems like swimming uphill. Note all the annotations in comments he has to give so that he can understand how his functions will compose correctly.

Underscore makes iterable the first argument, and function the second. The speaker thinks this is backward, since making function the first argument and iterable the second gives you automatic currying. This is the order followed by Ramda (https://github.com/CrossEye/ramda), as well as Python's standard library (https://docs.python.org/2/library/functions.html#filter)

If you watch the first two minutes, he explains what the talk is about.

But I'll save you two minutes: Underscore describes itself as affording Functional Programming, but in fact.. it doesn't.

He then goes over several examples of common FP patterns, like currying, composition, and functors, and shows how underscore fails in these regards, and how you can do so much better if a few things were done differently in the library.