I'm familiar with the reservations systems the hotel brands use, and I have yet to come across a situation where the same hotel room is being offered for cheaper on a third party website. It's also their official policies, and as I said, it doesn't make sense to me that they would advertise that and then not set a simple technical limitation in their reservation system to prevent anyone from being lower than them.

Yes, for some specific hotels that don't belong to a big brand, it makes sense for them to price discriminate so that the person willing to pay $500 doesn't end up paying $200, but on the level that the big brands operate, it doesn't make sense to give up 15% of gross revenue for that reason, especially when they have better ways to price discriminate by tiering their rewards members and the rate at which people earn points.

Edit: As jonknee pointed out in another post, the ihg.com price for Kimpton Monaco in Seattle is higher than the hotel tonight price, and by quite a bit, and there's a carveout in the best price guarantee for rooms reserved within 24 hours of checkin. So I guess the best price guarantees aren't really worth much...