Does Backbone.js have a hard dependency on jQuery?

Try to use jBone, this is library for DOM manipulations in modern browsers, jBone developed special for Backbone, only 2kb gzipped and jQuery like API.

Backbone is now even easier to decouple from jQuery.
Check this out:

Using Backbone without jQuery

You can use the Backbone.Model without jQuery, but Backbone.View will require either jQuery or Zepto, just like the docs state.

While Backbone.View does depend on a DOM manipulation library, ie you can't use vanilla javascript that is not organized in to a library of some sort, you can configure Backbone to use whatever library you'd like.

See the following from the Backbone source:

// Set the JavaScript library that will be used for DOM manipulation and
// Ajax calls (a.k.a. the `$` variable). By default Backbone will use: jQuery,
// Zepto, or Ender; but the `setDomLibrary()` method lets you inject an
// alternate JavaScript library (or a mock library for testing your views
// outside of a browser).
Backbone.setDomLibrary = function(lib) {
  $ = lib;
};

Calling this method will allow you to use whatever library you want.

For example:

Backbone.setDomLibrary(myCustomLibrary);