You don't need CI if you don't have any tests...

You don't need anything if it doesn't matter whether the software works or not.

Sure you do. Lots of projects are using compiled languages, even if the developers for whatever reason have decided to not write tests (lazy? lack of competence? decided tests wont benfit the project).

These projects too will benefit from a independent CI-service validating their commits.

Or deployment.

TBF this can be integrated to a pure-git setup e.g. Python had buildbots (on mercurial but samesies) long before it moved to github or whatever.

Generally speaking you'd set up your own "synchronisation channel(s)" for such tools e.g. mailing lists.

"pure git has all the features you need, if you use pure git alongside all these other third party tools"

The point of github / gitlab / etc is to have all the tools in one place, nicely integrated

I most certainly agree.

You don't really need gitlab/github for that, you can automate any kind of tests/build/deployment process with pure git hooks.

I personally feel gitlab/github are useful from community and collaboration aspects - there is nothing I couldn't do without github on my solo projects from purely technical point of view, it's strengths are issue tracking, pull requests, wikis, distribution of code.