I have heard both, but have not actually seen it for myself. On Costco Travel website in FAQs it actually states it’s eligible to earn WoH points.

I would check first with Hyatt by making a cancelable reservation than calling Hyatt for them to check to see how it shows up on their end.

Interesting that under FAQ they call out World of Hyatt loyalty points and not any other brands.

https://www.costcotravel.com/Info/Hotel-FAQs

Q: Can I earn hotel loyalty points?

Room-only bookings are eligible for hotel loyalty program points with World of Hyatt. When you are in the Finalize Booking step, simply place your loyalty number in the field marked “World of Hyatt”.