It all has to do with this line: "By providing syntactic sugar for these use cases we will enable a new class of "virtual method" library, which will have usability advantages over the standard adapter patterns in use today."
Essentially, you write functions that provide a certain behavior without being coupled to a specific class, so you can have a 'map' method that is called the same way no matter what you're mapping over.
Right now functional libraries like static-land, ramda, sanctuary, etc. are working hard to provide these functional primitives but it's still painful to write readable code that uses these.
It's also worth pointing out that directly importing methods here is more efficient than importing entire classes if you're not using one of those emerging build systems that does very good tree-shaking.
This is the closest JavaScript is going to get to letting you write your behavior as a suite of functions separately from specific classes—it's like a step removed from typeclasses.