Because when it's said by a company that is in the business of buying and selling domain names and has sold multiple domain names for big dollars it's BS. I operate in that industry, this isn't speculation on my part.
Otoh, if I approached Marissa Mayer to buy a domain she owned I wouldn't consider "I don't want to sell" "BS".
If I approached McDonalds and wanted to buy "hamburger.com" from them (hypothetical) and they said "don't want to sell" I wouldn't consider that BS.
But this company, that is their business (domains). It is BS.
Lastly, If your vanity domain name is (hypothetically once again) "nowblog.com" maybe you aren't interested in selling it. If someone offers you $100,000 you might be interested, correct?
how is that bs ? I have a vanity domain that I'm not interested in selling.