If you look closely, it says to inquire about other length stays (or something along those lines.) Testing on the site itself, under the current promotion, if you try to do 2 nights, it won't let you (the check-out date will revert to 6 nights), but if you do 3 nights, it will stick.
It's tricky for sure and much more subtle than they've done in the past (though, I'm sure somewhere it lists the actual requirements, though I couldn't find it in a pretty quick search on the site).