Cynic in means says it's two main things:

1. Effectively serves as a walled garden for copilot to make sure that competitors can't use their data (your code) in coding assistants, knowledge discovery, etc.

2. Ensure that competitors who are using github are maximally mineable so they can easily discover and implement their solutions to eat their lunch (much like OpenAI's strategy here)

This is covered up by the excuse of performance, but it's fundamentally no different than the loginwalls we saw go up around twitter and reddit

... except this time done by a big industry player who LARPs as part of the open source community but is positioned to cannibalize it from within.