Other people must have very different setups to warrant this kind of additional overhead and tooling.

In our repo local testing is first. You check out the code and can run the tests. The github runners do the same steps the readme encourages humans to do when running tests locally.

Also I'm not sure what duplication the author is avoiding with their DSL as Github actions can be broken into components and reused and parameterized. Of course if you don't like YAML you're never going to be happy until you've wrapped it in your own layer of tech debt.