AFAIK you don't necessarily need to use HAML, SASS, and CoffeeScript in order to use Backbone. They are tools that you can "plug in" to your web application to make you more productive, but I think they deserve separate treatment. And you need a server but it doesn't have to be Sinatra or anything written in Ruby. But Sinatra is as good a choice as any.
If you're going to study Backbone, it might be better to just study Backbone, using standard HTML/CSS/JS instead of HAML/SASS/Coffee. That would simplify the prereqs a bit and make it a little more accessible. But on the other hand, if you're at the point where you have a practical need to learn Backbone, it's sort of assumed that you're a web developer and should already be familiar with HAML, SASS, and Coffee. So I guess it's a judgement call by the author.
If you're going to study Backbone, it might be better to just study Backbone, using standard HTML/CSS/JS instead of HAML/SASS/Coffee. That would simplify the prereqs a bit and make it a little more accessible. But on the other hand, if you're at the point where you have a practical need to learn Backbone, it's sort of assumed that you're a web developer and should already be familiar with HAML, SASS, and Coffee. So I guess it's a judgement call by the author.