Fair, and ill edit my post accordingly!
There are teams that are allowed, and within those companies are teams that are directly related to foss projects because those companies are in the foundation or supports of the foundation. However, thats doesnt mean every (product) team in the company is allowed to or that they can do or change whatever they like. Its a complex mess
Not saying youre wrong, but you took the wrong project as an example hehe.
Visual code is not open source. Its core is, but visual code isnt. The difference is what visual code ships with, on top of its core.
Its like saying chrome == chromium ( it isnt ).
Visual code comes with a lot of features, addins and other stuff that isnt in the core.
.net debugger for example, is not found in vscodium ( build of the vscode core ). And there is more stuff i cant think of now but have come across. Source: been using vscodium for a few months instead of vscode