Long story short: they messed up the assign-reviewers.yml workflow, allowing external contributors to merge PRs without proper reviews. From this point on, you're fully open to all kinds of bad stuff.
That’s not what happened at all
The attacker did not need to merge any PRs to exfiltrate the credentials
The attacker did not need to merge any PRs to exfiltrate the credentials
The workflow was configured in a way that allowed untrusted code from a branch controlled by the attacker to be executed in the context of a GitHub action workflow that had access to secrets.