Similar they do with the VScode, the rumor is. They only add the telemetry in the binaries.
When we [Microsoft] build Visual Studio Code, we do exactly this. We clone the vscode repository, we lay down a customized product.json that has Microsoft specific functionality (telemetry, gallery, logo, etc.), and then produce a build that we release under our license.
When you clone and build from the vscode repo, none of these endpoints are configured in the default product.json. Therefore, you generate a “clean” build, without the Microsoft customizations, which is by default licensed under the MIT license
Source: https://vscodium.com/#why-does-this-exist
Just a heads-up warning - add-ons in VSCodium can't be installed from directly Store, you need to download and install them manually.