Alright then. In that case, take into consideration what exactly DLC is. Downloadable content.
Mewtwo is not limited, special edition downloadable content. There is not a limited amount of Mewtwos available for download, whether you have payed for him or already gotten him for free. Assuming there were an infinite number of copies of SSB4 in existence, there will also be an infinite amount of Mewtwos available for each and every copy, because DLC exists for everyone until the company decides to discontinue it.
So basically, the quote you read about some guy saying that they have "sold out" of DLC Mewtwos was a joke, presumably based on the rubbish availability of amiibos.