This isn't Backbone.js -- it's an alternative. Different codebase, different function names, partially overlapping API.

At the top of the FAQ, he answers some of the comparison questions: http://maccman.github.com/spine/#h-faq

The bit about it being smaller is pretty misleading (especially when comparing 2k to 3k) -- it may be 500 lines instead of 1100, but the 1100 are heavily commented, and the 500 are entirely comment-less. It also doesn't include any of the collection mapping, filtering, and aggregation functions, which contain a large part of the usefulness of Backbone when used with real models.