What exactly does this mean? I'm assuming it doesn't imply I can request certain makes/models with 24 hour notice?

No. It means that even if the location is sold out you can still book a vehicle for anything up to a full-size. Just login to your account and the location will go from sold-out to available.

A few years ago it was 4 hours notice, then they bumped it up to 48 hours which made it far less useful. 24 is at least somewhat better than 48 hours...

Valet isn't a benefit of PC as such - it's a "4 times a year" thing if you have PC status. If your thinking is that because you're going to get PC status on your 16th rental, I would not presume that'll automatically give you a free drive-back, and in fact I'd be surprised if it did.

I've never taken them up on it (5* so I still have to pay) but I usually receive an e-mail about 12 hours prior to my scheduled return asking if I'm interested. There's a click-to-purchase button in the email. I'm assuming you'd receive the same e-mail, but would have it comped.