A COUPLE of things come to mind:
1. Flynn is an admitted LIAR and he was fired from his post for lying to The Vice President and President. He's got an axe to grind.
2. If he doesn't promise to testify against The President, His Family, etc., then he's got NO DEAL with Mueller, so he'll come up with SOMETHING - true or not.
I thought the judge said he just has to provide information that leads to someone being prosecuted. No one mentioned the President.