GitHub uses fairly long-lived sessions. "sudo mode"[0] on GitHub, where it asks for a verification of the 2FA is only for sensitive actions, which PR merges are not. So a cookie-stealing attack can easily merge PRs for quite a while.
And 2FA isn't a requirement for a PR merge afaik, Except via org-wide enforcement? So the guarantee is lower - the commit was merged with a valid session token.
[0]: https://docs.github.com/en/authentication/keeping-your-accou...