Since the YTCA doesn't recognize different sizes within the breed like Poodles, for instance, there is no such thing as a "teacup" Yorkie. The standard only calls for Yorkies to be under seven pounds.
I think everyone jumps on the use of the term "teacup" because it suggests that whoever is using it is either unaware of the breed standard or is deliberately trying to mislead people. It's a huge red flag when you see it on a website.
I just cringe whenever a see a picture of a poor puppy crammed into a teacup to show how small it is.