Great question actually.
I'm basically
paraphrasing my post on it.
@
Tama has constantly referred to it as a rumor and not a leak, so I've done the same.
To be honest it could just be a word choice.
Though, if I had to guess the reason it's a "rumor" and not a "leak" is because the information was first told via a reliable source. If they had given more hard evidence (Images, video, documents, etc) in direct relation to the Shovel Knight in Smash rumor, Tamaki might have called it a "leak". They are a reliable source of information, which is the whole reason we are entertaining this rumor. If it was just a random 4chan rumor, I personally wouldn't give it the light of day.
In the end the "rumor" rests on a couple of noteworthy collaborating sources...and now some strange coincidences which point to the rumor being potentially true (Yacht Club Games no answer on the rumor, saying there could be multiple amiibo, etc). However, until we find hard evidence, it's best to call it a "rumor", as we are not 100% sure.
A lot of what we label as "leaks" should actually be labelled as "rumors" if we want to get into semantics.