Looking into this particular situation, VSCodium is just building the from the same source, only, without the telemetry functionality and MS logo included. They include a quote from a current VS Code maintainer that says:
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.
So it seems that it should be the same. I haven’t tried it out myself though.