It is somewhat magical to me that these processes can be as selective as they are. Coffee beans must have hundreds if not thousands of organic compounds in them, and somehow something as simple as co2 (or the other solvents) picks up mainly caffeine. Somehow that is very counterintuitive to me.
I was interested and looked at the actual patent: https://patents.justia.com/patent/4348422 (there seem to be multiple patent documents, but this one adds some explanation), and he writes "I have now surprisingly discovered".
https://tastydecafs.com/blogs/learn-about-decaf/co2-decaf further explains
"The story of C02 decaffeination goes back to 1967. It was then when a chemist at Max Planck Institute named Kurt Zosel stumbled upon an interesting discovery. Zosel, like many other chemists, was using high-pressure C02 to remove individual substances from other mixtures."
It must have something to do with caffeine being an alkaloid, while coffee overall is acidic. So I suspect that this pressurized CO2 is able to dominantly remove such alkaloids... I leave the details to a chemist :)
Extraction and separation are a function of time, temperature, and solubility, not just polarity. It's easy to target solubility limits for certain compounds, which why using a Soxhlet extraction method could be effective for at home decaffeination.
I've always assumed that's why steam extraction for espresso doesn't extract more caffeine than drip, and why a Madras coffee decoction isn't ultra-caffeinated concentrate.
