It's not a bad idea generally, and it's definitely a great idea for the stated accessibility goals. I think they mostly just need to document the right .editorconfig settings to keep things readable in most editors, or even provide a tool to auto-update .editorconfig, perhaps.
Relatedly, I wonder if some of the remarked upon issues with Github could be fixed on Github's side with increased .editorconfig support in repos.
Finally. Spaces are so annoying when it comes to aligning things without the help of an IDE. We use tabs at work and it’s awesome. It doesn’t matter what your tabs are set to (I use 3 spaces, fwiw) and everything is magically aligned. It requires a well-thought-out style guide to make it work I think.