Buried at the bottom of the article is a link to this github issue that explains VS Code's policy, which is apparently to make beta-quality APIs available to one or two extensions first to get feedback, then make it publicly available later. The extension author gets access to the API early but accepts that it's non-final and subject to changes or breakages. This appears to be what happened with InlineCompletionProvider which is one of the APIs Codeium complains about.
Sure, it's very convenient for Microsoft to give early access to another Microsoft product but it's not the worst policy ever to get feedback before everyone in the world starts developing against API and it becomes harder to fix design problems.
I wonder if Codeium has requested their extension to be selected for this status.
If you expected someone's fledgling project that's just getting started to be as polished and refined as something that's had thousands of dev hours over the last 5-10 years then that's just bad expectations management. It will get better.