You should read Robert Barnes (lawyer) on Twitter.
https://twitter.com/Barnes_Law/status/1075120487728959488
https://twitter.com/Barnes_Law/status/1075115899500126208
He was trying to get Mulehead to say he would NOT consider treason (which he did), and take stuff “off the table” that Flynn could be threatened with.
But the fact that Flynn refused to retract his plea should tell you that Mulehead indeed has something on him.
It doesn’t take even a third-rate lawyer to get Mueller to take treason off the table. Flynn couldn’t be charged with treason even if he sold nuclear secrets to Turkey.