I've just tried many dates for Sign in Sept and Oct, and I always get the disclaimer notice just above the box you click to make the reservation.
If I select the "best available rate", I do not get the disclaimer box, and the reservation is cancellable until 4pm local time on day of arrival.
This disclaimer is only when booking with points.
The last reservation I made for Sign was on 6/18, and did not have this disclaimer notice. Perhaps this is a very recent change.
When did you book your reservation?