Most likely only a small fraction of the people who were going to make a purchase heard this news and only a fraction of them decided to follow through and purchase the next day.
It’s very easy to postulate that people would go to another site (e.g. Amazon) immediately instead of waiting around to see if the Costco site would recover, especially since most BF deals are limited by both time and quantity. That alone makes the loss estimates feasible.
This is the part I was referring to as "not actually true". Fact: the site itself, which was loading although not working entirely, displayed a big banner saying this during the problem periods. It was hard to miss.
The rest is indeed just speculation either way. I doubt the losses are as high as estimated, but there's no way to say without inside data.