compromising a keypair is a huge win. lets you impersonate the domain. shorter validation periods = smaller windows of compromised situations.

basically the smaller you make the window the less manual intervention and the less complicated infrastructure gets. currently TLS systems need a way to invalidate certificates. get them down to a day and suddenly that need just disappears. vastly simplifying the code and the system. 6 days is a huge improvement over 90 days.

I'm far from an expert on PKI, but isn't the keypair used for the cert used for key exchange? Then in theory, if that key was compromised, it could allow an adversary to be able to capture and decrypt full sessions.

The key pair you're thinking of is just a singular key for a block cipher. That key needs to be generated/transmitted in a secure manner. Meaning that its security is dependent on the cert. The expiration time of that cert is what they're aiming at.