You're not missing anything. It does use GitLab's mirroring API. However, GitLab doesn't have any global mirroring settings, you need to set up each repo individually. The tool just saves you a huge bunch of clicking around and copy pasting auth tokens. Useful if you have a big collection of repos you'd like to mirror. And if you'd like every new public repo to be mirrored automatically.
Meaning, a repo can be hosted on GL but still benefit from better exposure and disoverability of GH.