I'm not sure how it's possible but it's definitely not rare. Lodash' methods are faster than their respective native implementations (Lodash `map` > `Array.prototype.map`).
As a general answer, doing something simple in pure JS can be more efficient than the overhead associated with making a native call from JS. The costs are going to vary depending on how the JIT handles things. That's probably the best general reason though as to how it's possible.
As I understand it, native promises don't make native system calls. They are still pure js. They are just implemented differently from Bluebird which optimizes for the most common use cases.