From scratch means if I am on a fresh install of whatever OS they are targetting and have a compiler (compiler, not build tool, so rustc, not cargo) of the language they use I can build the project without having to install any dependencies.
Seems like you arbitrarily draw the line. rustc but not cargo?
Even if you don't use any libraries, cargo is a useful tool. And if you reject tools, what editor/IDE would you code in?
I'm not in the construction business, but I guess if you build a house from the ground up, that means you're not building on top of some existing structure, but you might still be using prefabricated elements.
I'd put the line here: whether ground work is used that was laid for editors, as opposed to general programming. If the thing that they are making (an editor) does not contain editor-specific prefabricated parts, then all the editor-specific work was still made from the ground up.