Nope, this is inaccurate. We've stayed at the Polynesian many many times. The sign may have recently changed but forever it just said the price and that it was good for unlimited refills.
When the Polynesian first started selling mugs, we bought a couple or more each trip. We have pink ones by Thermos which were the BEST, pink ones that are similar to the current ones except in design, some with celebrations, etc. We always bought new ones each trip until one time I heard a CM telling another patron they were good forever. For a trip or two, we did take ours back but in general I think that is a pain.
After I started reading the DIS in 2000 or so I became intrigued with the controversy. So every trip after that I specifically looked at the sign. It never did change to length of stay. We haven't stayed at the Polynesian since January of this year and I can't swear it may have not changed by then. But it did not say that in 1997 or for years afterwards.
Nonetheless, there have been posts with letters from WDW saying that they are length of stay now.
And boy, can I relate to them tumbling out of the cabinet!
Maybe I should take one of our old pink ones back.