This article you posted from Reuters was published on June 12th. Since then, Donald Trump has returned to some of his old rhetoric from during the 2016 campaign, and he has said that the only way he can lose the election is if it is rigged. Since this Reuters article was published, he has also refused to say if he would accept the election results if he loses. Donald Trump will only say that if he loses, it has to be because the election was "rigged". Donald Trump has never provided any evidence in support of mass voter fraud.
From The Hill and posted on August 17th:
"We have to win the election. We can't play games. Go out and vote. Do these beautiful absentee ballots, or just make sure your vote gets counted. Make sure because the only way we're going to lose this election is if the election is rigged. Remember that. It's the only way we're going to lose this election, so we have to be very careful." - President Donald Trump, in a speech in Oshkosh, Wisconsin
That has nothing to do with Hillary Clinton and nobody is putting words in Donald Trump's mouth either. Trump said very similar things four years ago. I think this probably has more to do with Trump's childish need to protect his ego, and build in an excuse in case he does lose. I don't think he would refuse to leave office if he loses, but with him, you never really know. There is no presidential norm that he won't violate to serve his own self-interest. He is narcissistic and self-absorbed.