Uggg. This should be called Singool.coffee, not Singool.js. Coffeescript is not javascript, just like Clojurescript is not javascript. It's another language that compiles down to Javascript. I really wish libraries built in Coffeescript could use the extension .coffee instead of .js. I'm glad the Clojurescript and Emscripten don't use misleading project names.
It is clearly stated in the documentation that you are not required to write your app in CoffeeScript if you want to build with Singool.js. The choice is totally yours.
how is it lightweight if it is built on top of Backbone.js?
Lightweight in the sense that Singool.js utilizes the features existing in Backbone.js without trying to add too much to it. What it does is provide more of a structure to your apps.