Seems they no longer use this method. Instead they use `CHANGELOG merge=union` in a .gitattributes file at the root of the repository. That means that conflicting merges take changes from both sides of the conflict. It's what you usually want with a changelog.
Sumup:
> At GitLab we solved the above problem by adding a 100 lines with just a hyphen placeholder at the top of the changelog. People can insert their entry at a random location in the changelog. There is still a chance of conflict when two merge requests change the same line but it is greatly reduced. It looks a bit strange to have these empty lines on top so we added a comment to explain their purpose..