You are mixed up. The unproven accusation was made by Trump as seen here: Trump:
"All of us here today do not want to see our election victory stolen by emboldened radical-left Democrats, which is what they're doing. And stolen by the fake news media. That's what they've done and what they're doing. We will never give up, we will never concede. It doesn't happen. You don't concede when there's theft involved".
Please note above that Trump is using definitive language when stating that the election was stolen, and therefore Trump is NOT expressing that it is his belief or opinion that it was stolen.
As for what does count as factual evidence, at the same speech Trump admitted publicly his intention to overturn the official election results by trying to get Pence to not recognize the official electoral count so that Trump could remain President. And that does not qualify as an unproven accusation; it qualifies as factual evidence of an attempt to overturn the official results of a Presidential election that Trump had officially lost. Here are his own exact words admitting to that intention:
Trump: "I hope Mike is going to do the right thing. I hope so. I hope so.
Because if Mike Pence does the right thing, we win the election. All he has to do, all this is, this is from the number one, or certainly one of the top, Constitutional lawyers in our country. He has the absolute right to do it. We're supposed to protect our country, support our country, support our Constitution, and protect our constitution. States want to revote. The states got defrauded. They were given false information. They voted on it. Now they want to recertify. They want it back. All Vice President Pence has to do is send it back to the states to recertify and we become president and you are the happiest people".