It clearly states in the article that though VSC is open source and you can build it from source, there are opaque components that are part of the VSC installer. The article claims that the installer installs telemetry systems, though they can be opted out of. However, the project owners do not trust Microsoft to honor the 'opt-out' part if not now then in the future.

So they have created a fork of the project.

Why in the hell would anyone trust them?

That makes no sense. You are moving from trusting Microsoft, which has actual skin in the game, to random people on the Internet that don't.

Plus Microsoft. Because it's still their code underneath it all. So it's not like VSCodium actually solves anything...