To take an example of a large OSS Project hosted on Github: The Microsoft .NET Core framework[1].
If I want to contribute a code change - their guidelines require me to submit a pull request on github.
If I want to submit an issue - their guidelines also require me to log the issue on github.
This is not unusual for most OSS projects which use Github as their central collaboration point.
Today that's all A-OK for Chinese devs. The scenario discussed is if the trade-war extends, and either the US or Chinese governments block access to Github for Chinese users.
Now, perhaps it's easy enough for someone to set up a mirror copy of everything in China - but that's going to limit or block collaboration. Bug fixes, changes, issues - they're not going to get across as easily. I can't email a pull request to the .NET CoreFX team saying "Hey, I fixed this".
This is going to lead to fragmentation of popular OSS projects, and will be an overall net-loss for everyone.