So I went to the original Nintendo Everything article and this is what it had to say.
Retailer wouldn't know about Smash DLC because, besides the amiibo, the DLC doesn't go to retailers. amiibos do go to retail and there have been leaks of amiibos through retailers (like Shovel Knight, in fact). Therefore, the rumor was about the existence of the Shovel Knight amiibo, not about the Smash Brothers DLC. I think the guy from Unseen 64 and even the tipster assumed this meant he was coming for Smash. I think this is a case of cognitive bias.
I agree. I think this was all revolving around the Amiibo, but supposedly Tamaki had a different source other than just retailers. Nintendo Life, on the other hand, only cited retailers. They probably heard "Shovel Knight Amiibo" from other retailers and assumed it was the exact same listing as the one that said Smash.
Its also notable that people tweeted Yacht Club after the fact and again tried to claim Yacht Club lied about this update, but they stated quite clearly after the fact that they said the Amiibo listing was fake because it was a Smash line Amiibo, not because they weren't producing an Amiibo.
Before the Amiibo listing was true I noticed as well that they primarily cited "retailers," and I kept thinking "Ok, what? Retailers wouldn't know anything about DLC, but an Amiibo they would." Unless it was specifically a Smash Amiibo line, but we had no evidence that was the case, and it was extremely unlikely to be the case because we didn't yet have confirmed Ryu, Roy, or Lucas listings (and its now been proven to not be the case at all).
I don't think we will get confirmations of Ryu, Roy, and Lucas until the remaining DLC characters are announced, either, so they can make up the last wave of the Smash Amiibo line.