That's the problem though. In principal, if they only use pure-git operations in the process, it would be easy to move at any point. But if they leverage the entire Github platform, i.e. PR discussion, issue tracking etc, then important parts of the legal history will be bound to the github platform.
And that's not great. Github is free for public projects for now, but as with any publicly traded tech company, they're able to change the terms of use of their product at any time to suit the needs of the company.
When we're talking about enshrining the laws of society, we're talking about a very long-term proposition. Technology companies pivot and or kill products all the time. What if we decided to record legislative history on Google Flow?