Building the open source repo gets you something which isn't VSCode. It doesn't install a lot of VSCode extensions, therefore you shouldn't call it VSCode and shouldn't say VSCode is FOSS.
Edit: to be clear, I think Microsoft should offer VSCodium themselves and state clearly that they take that, bundle it with other stuff, and are calling that blob VSCode.
The instant they do that I'm moving to Theia or something else (zed?). MS is not to be trusted, as outlined by this article.