That sounds like you would need to do a whole lot of work before you could start writing code that is useful to the business that hires you. If your first job out of uni is working at a company that has built a site in ruby on rails, you shouldn't refuse to handle tickets until you've reproduced rails. Also, if you want to learn backend web development, it is far easier to first work through the djangogirls tutorial and learn how django works and then build a stripped-down version of it.

I would replace it with "Don't add a new framework or library unless you can get a clear mental model of it"