I'm not sure about gitlab, but on github this is a distinct feature where you post a comment wrapped in three backticks and the language as suggestion. It'll format it as a diff that the author can one-click create a commit from.
Services like GitHub and GitLab support preventing changes that overwrite a repo's history, in addition to tracking history even when you force update a branch.
You can revert and even force-update the branch to rewrite it's history, but GitHub still tracks the old commits and even lists in the PR the events that rewrote the branch history.
It's neither new nor novel. Even GitHub allows anyone to post a commit on a feature branch, regardless of who created it.
It's not a GitLab or GitHub feature. It's a Git feature: the ability to post a commit to a branch.