People originally gave it "credibility" because the leaker leaked a few new alternate colors before they were shown.
It was two colors, the color that is Mario's new blue team color (I think it's blue team, anyway? It was in the E3 demo) and Bowser's new color from the same E3 build. The source remade these colors in Brawl (using BrawlBox) and had uploaded them to an imgur that dates before E3. There was a third color he made that hasn't been confirmed yet, which is basically a blue Bowser (not the old blue Bowser).
And then there's another person who came out about hearing from a friend that Duck Hunt Dog and Bowser Jr. were in. This person is Vaanrose, who actually lives in Redmond, WA (where NoA is). I forget the exact details Vaanrose said but the jist of it was just that he was told about Robin/Lucina (forget which one) before the trailer came out, along with Duck Hunt Dog and Bowser Jr. so he didn't believe what his friend told him until the Robin trailer came out. Of course, that doesn't lend any credibility to it (anyone could say that) just paraphrasing what he said.