Most orgs like having a single source of truth for their projects, though (for instance, the Linux kernel GitHub repo is simply a mirror). In fact, if anything, this project could set up mirroring so repos are mirrored from GitLab to GitHub, etc.
But then, the Linux kernel really have multiple sources of truth, depending on what version you are talking about (Linus branch, mainline, vendor forks, etc)
The main point isn't about the source code, it's about all the rest : bug report, discussions, planning..
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You know git is distributed? You can keep all your repos both on GitLab, GitHub, as many local machines as you want... etc. all at the same time.
Most orgs like having a single source of truth for their projects, though (for instance, the Linux kernel GitHub repo is simply a mirror). In fact, if anything, this project could set up mirroring so repos are mirrored from GitLab to GitHub, etc.
But then, the Linux kernel really have multiple sources of truth, depending on what version you are talking about (Linus branch, mainline, vendor forks, etc)
The main point isn't about the source code, it's about all the rest : bug report, discussions, planning..
I think is more about the political statement of leaving GitHub rather than the practicalities of code management.