It was misleading by me also. As mentioned, terrible work day yesterday, so I was writing angrily. Ironic that I accuse the security people of being bad at explaining, then turn around and explain stuff badly myself...
Ill TRY to make it up by explaining it as i understand this.
This is not adding the pin functionality, I was just focusing on the flow that happens when you get the Fido2 challenge flow on a webpage. That functionality is already there. Thee new thing is the 5.8 ads support for a second roundtrip/flow for signing stuff in browsers.
Right now, a YubiKey does only one thing in browser: signing a "login challenge string" from the webpage (FIDO2 flow). This is just slightly simplified: (random number + domain + some other shit). Fingerprint that string and return that cryptographically signed result to the site, as proof that "Only I could have signed this login challenge."
Right now, that's the only supported back-and-forth flow for signing anything with a YubiKey vs. a browser.
The new thing is that YubiKey wants a second flow, where the browser sends a request to sign any arbitrary text. So the page contains a contract, converts that to a fingerprint, the browser sends the fingerprint to YubiKey, YubiKey cryptographically signs this, and you can post the resultant string anywhere (public key cryptography magic), and that proves that the person holds that hardware key signed that exact input string (the contract).
So if/when Chrome, Safari, Firefox, and more accept this standard, you could click a button below a contract on a webpage, and the back-and-forth dance with the YubiKey would result in a string below that contract being cryptographic proof that the exact contract above is signed by the person holding one specific key.
Notice the lack of any AI-related shit in this functionality, yet the article reads 5.8 is a revolution for security in "the AI era"