The DX is the developer experience. It follows a similar idea. How do you feel about using git? What about Visual Studio Code versus neovim? How do you like using Elm? What don't you like about.
Think about all the libraries or frameworks you use. Some of them clearly offer a better experience than others, these are the types of things that affect the DX.
Take documentation for example, a piece of tech (whether it be an IDE, library, or CLT) with proper documentation full of examples, usage, and recipes is extremely nice. I would favor tech that has better documentation over others, documentation is just one part of the DX. Not the only part, but hopefully you can start to see what can increase or decrease the DX.
To me, how I feel about using (working with) git is git's UX.
How I would feel working on git (were I to do so) would be git's DX.
Now, for a project that has git as part of its expected dev tooling, how I feel about using git in that particular context would be part of that particular project's DX.
You may think that, but it's a standardized term already.