When your entire project is sitting on GitHub like nearly everyone else out there and something goes wrong, don't be surprised when one has to tweet at the CEO of GitHub if the repository is falsely flagged for 'some reason' or if your GitHub actions, pull requests and packages are experiencing a degraded service.
Gitlab self hosted is fantastic, but it's far from 'set it and forget it' maintenance. There's significant technical overhead to keeping it updated, secure, migrating its database with major releases, scaling out the underlying hardware as project demands increase, etc. Moving to a hosted platform like Github solves those problems and gets them back to being productive on their project.
The authors are really lamenting that they have to give up control of their project's source code and tie themselves to a new commercial offering (something that history has shown us over and over will inevitably lead to more technical debt in the future as platforms fade--just ask any former sourceforge, google code, etc. user).
I don't blame them--it's sad to see that the state the art in self-managed OSS source code never progressed much beyond "become a domain expert in server operations and kludge together a suite of tools with wildly different UI, management and operations; also, documentation is non-existent or wildly out of date".