It looks like they have a very specific and unique build process which they really should handle with something more customizable like Jenkins. Instead they're using something that's really intended for quick and light deployments for intense dev ops setup.
I really like GitHub actions, but I'm only doing very simple things. Don't call a fork bad because it's not great when you're eating soup
They either just write a long blog post about how they can't screw in nails with a hammer.
Or they leave their security rules wide open and about half the comments are like, we need tools which stop us from doing stupid things.
No other industry works like this.
What it really does is fire off a WebHook. Repository custom properties and the name of the action are properties that are included in the workflow_job webook. With this you can do anything you want and you're not at all constrained by YAML or runners.
If they had, we'd be reading a different article about how terribly complex and unintuitive Jenkins is.
CI is just a very very hard problem and no provider makes it easy.