1. Apple Mail has had this feature for the last 20 years. If macOS users to whom that feature is of interest are only now realizing it exists, this speaks volumes for Apple’s documentation and marketing.
2. The page says you can use AppleScripts as rule actions. It never tells you how. No sample scripts to use or learn from. No links to the documentation you’ll need to write your own.
Here’s a better third-party introduction from 2005(!) which includes a couple rule action examples:
http://preserve.mactech.com/articles/mactech/Vol.21/21.09/Sc...
TBH, after 6 years rusting in maintenance mode, all its documentation long buried in archive, and the department responsible eliminated entirely, I am surprised Apple would still want to draw any attention to AppleScript at all. They only set up the unwary for disappointment later, once they discover for themselves the platform is already a walking corpse. That is no way to sell product. Alas, post-Jobs Apple cares little for joined-up thinking, or even motivation to do a half-decent job.
Let’s hope the Shortcuts team can eventually make a half-decent success of their product, because AppleScript automation has zero future. That said, if Ventura Mail still can’t use Shortcuts as rule actions then it doesn’t bode look great for them either.