"Assuming a ReactOS developer used Microsoft / Github Copilot to work on this codebase"

No need to assume. It's a certainty: https://github.com/reactos/reactos/pull/8516

"if Microsoft attempts to sue (themselves?) over their own Copilot tool injecting their own copyrighted code into a user's codebase"

Such an attempt can't make sense, given that the model used by ReactOS is in Microsoft's control and thus Microsoft alone is the one responsible for the model's behavior. They won't sue, thus much is clear.