That sounds about right for how it works now. The system no longer turns a blind eye to reservation numbers/names/check in dates. It takes your reservation number, your name, and confirms that you are 180 days from check in. Once you confirm this, it opens up your 10 day window.
The one oddity/glitch is this:
You said that your 180 days is Sunday. So on Sunday you can log in and reserve for 10 days regardless if you're staying 10 days or not. If you log in on Monday, you can also reserve for 10 days from Monday.
I am not sure how long this goes on for. So I don't know if someone could log on today with their December reservation number and reserve through the end of January.