Welcome to Flyertalk Marty82. It's possible the hotel is fully booked but had released rooms to hotel pronto. I am not familiar with this site.
Do they have real time availability and confirmation or do you book and they contact you afterwards to confirm the booking? They might be offering it but might not have it.
What date are you looking for?
Will you get points from a non-IHG site- No.
Will PC price match, No, not if they don't have availability.