The CVE looks relevant to me. If you're trying to get a package from a private repo, and someone publishes version 999 of that package on a public repo, then you're executing their code instead of your own. Just because the pip maintainers refuse to fix it doesn't mean it isn't a security vulnerability that could lead to remote code execution and needs to be controlled for in some other way.

One could argue that this is not an issue with pip, the software, but of the index used. I mean, if you control both index and extra-index there is no problem (and one solution to this is to use your own mirror with a set resolution order). This could very well be addressed in PyPI, for example NuGet allows to reserve package prefixes.

We also do not create a CVE for curl because you can use it to download the wrong bash script. If this was an alert for suspicious usages of pip instead of pip itself, I’d be less critical of it.