If you read the website you will see that Git Pusshuten can do more than just do a simple "git push" like other solutions.
Git Deploy by Mislav makes the assumptions you work with: Passenger, Ruby on Rails, etc. Git Pusshuten doesn't make such assumptions. It allows you to deploy an application, regardless of whether you're deploying a Ruby, Python, PHP, etc application, to whatever web server by defining _your_ environment through modules.
Additionally, it allows you to provision and pre-configure your environment so you can be up and running with minimal effort and sysadmin knowledge. Other solutions will make you manually set up your environment by SSH'ing into the server to configure everything. Now, I am definitely not saying that that is a bad thing. I'm just saying that Git Pusshuten's _goal_ is entirely different from that of Git Deploy.
So what you're pointing out now would only work for people using Phusion Passenger and a Rack application. That's a limitation that Git Pusshuten is trying to avoid.