Based on what you posted, here's what was said before the Once Piece parts leaked:
If we then break it down, claim by claim:
The characters of Luffy, Shanks, Buggy, and Zoro.
We have seen two parts for Luffy, nothing from Shanks, one part from Buggy, and nothing from Zoro (unless the beanies would go to them, which I'm not sure about). Even then, they all seem to be popular characters, so it's not too hard to guess they could be included.
There were quite a few molds
Also something easily guessed.
Some parts made of soft plastic
Assuming that is referring to the arms, this is probably the best evidence going for it, but it's also not an outlandish guess either.
The colors were accurate
Was there a doubt that the colors would be inaccurate? Basically a nothing statement.
The legs had dual printing
No legs have leaked yet, so there's no way to confirm that yet. Though that also wouldn't be an outlandish guess.
The hats were well-made.
Most of what we have seen are the hats, but, again, is there doubt that that could just be a lucky guess. If we're getting new licensed characters, getting new molds for their iconic hairpieces isn't too outlandish.
In other words, most of it is guesswork that, largely, could have been easily predicted. I can maybe lend some credit to the guess of the arms, but, again, that could just be a lucky guess. There's not enough therefor me to fully believe that this person 100% has insider information, or has knowledge that Goku/Dragon Ball is coming. There is a chance, but I'm not seeing enough to really believe it (and even if Dragon Ball does come, the fist "scoop" is enough of a vague guess that a broken clock could be right twice, so I'll still remain skeptical).