So I called, and they confirmed a more charitable reading. The mail starts with "Like in years past your $300 annual travel credit will reset in December and you will start to receive statement credits for travel purchases." So, after the December statement closing, there will be $300 available through December 30th. Anything not used will be a statement credit. Then a new travel credit starts on the account renewal date.

So those of us with late in the year renewals don't lose anything and get a small bump. Someone with a January renewal basically gets a free $300.

I don't know if they were correct, of course, so I'm going to be watching my statements in December closely...