I disagree with the "always" in this statement. Sloppy, lazy private repos sure. It is possible to have them completely absent in any and all repos though I've only seen and been part of such an effort once, it takes a lot of work to make sure it happens and I have little faith in most companies following through with that.
However, what's more common is secrets in CI variables. If their GitHub was breached, they should be more concerned with whether the attackers had access to GitHub Actions logs or secrets.
I've worked in places where all the code was open to all teams, with the exception of DevOps because they had too many hardcoded secrets and never had time to clean up their mess. I get it, they really were spread thin... but it should have never happened in the first place.
There are always creds in private/company repos! Lots.