> This might be overreacting but is there a way to opt-out of Copilot using your code in open source repos?

I don't think there is a way to opt out if it is a public repo regardless of license, and Microsoft's copyright theory suggests that they wouldn't feel obligate to enxclude any code they got their hands on except under a specific NDA preventing such use; the use of public GitHub repos isn't based on legal constraints but practical convenience.