No, they didn't. It's Emacs. Every thing is dynamically scoped lisp. It's like if you could include arbitrary javascript in your vscode config and if you shadow any core function any code calling it will now use your function instead.
How is that not a fork? You have to maintain your function. Upstream can change in ways that your monkey patched function won't work; you have to maintain that going forward.
If you want to share your change with others, you have to ask them: which version of VSCode are you on, and give them the correct monkey patch which worked with that version.
You're doing everything fork-like except calling it a fork.
You know what? I was wrong, you're right. I thought this was a change to the core engine, but I looked again and see it's in the lisp. The user's ~/.emacs file can override this setting, so no hard fork is needed.
