I suppose only Michelin and Costco know for sure, but I am 99% sure that this is not true. I have been buying Michelin tires at Costco since it was called Price Club. I have gotten up to 90k miles out of them and I have never had a tire purchased at Costco fail.
Well, that is not quite true, I ran over a sharp piece of metal that cut a big gash in a Michelin XZX once about 15 years ago. Costco replaced it.
One thing I noticed about Costco - a year or so ago while I was waiting for my tires, I went down the rack looking at the country of origin on every tire. Every tire in the place was made in USA or Canada. With all the complaints I have read about tires of various brands that came from 3rd world countries, the “Made in USA” on Costco tires gives me a degree of confidence.