Appcelerator Titanium is more of a platform than a framework. I guess it's like comparing CGI.rb to Rails. The other major issue with Ruby support on Titanium is that Ruby 1.9 isn't supported, c exts aren't supported, and Threads aren't supported. In other words the UI locks up whenever you call out to Ruby. Bowline used to sit on top of Titanium, but I made it standalone for those reasons.
Its just a ruby container for webkit. So Appcelerator, or I think you are talking about their titanium container, is just like this.
The basic idea is that you bundle a browser, and then using your programming language of choice, you can create APIs that are available inside JavaScript.
So, you can also probably create functionality in ruby, that can then be accessed in JavaScript.