The supposed leaker has contradicted his own claims about the material, has refused to release evidence (like cryptographic signatures) that might prove whether the emails actually passed through email servers, and he is a zealous Trump booster, (supposedly) hid the information from the public for a year in order to maximize campaigns impact instead of sharing it immediately upon discovery, and none of the Republican Senators not Republican DOJ, who supposedly had this material for a year already, have mentioned its existence, despite having recently published reports investigating this exact issue.
The purported facts beggar belief -- that an extremely famous person took his laptop, full of extremely damaging material, to a repair shop and then simply forgot about it. That a Mac repair shop owner violated all ethical standards to spy on a customer laptop.
That after stealing the customer's laptop and data and finding supposedly treasonous data on it, he gave it to Trump's campaign team instead of the FBI.
There's no evidence that it's true and substantial evidence that it's false.