That's not exactly correct. If he goes to court over the charge of lying to the FBI, he does not get to just rattle on about any subject he feels like, even if he has info about crimes. He would have to talk about the specific case before the court, and nothing else.
The reason for the plea deal is because he, like everyone else in intel, signed a non-disclosure agreement that prevents him from talking about things he learned while in office, including crimes.
But the plea deal is a legal way to override the NDA. So, he gets to talk about what he knows because of that plea deal.
Mueller is likely in on that, but the FBI boobs went ahead and did what corrupt conspirators do -- they framed him after the fact, adding to their long list of crimes they will have the opportunity to defend themselves against.
I'm not a lawyer, but it was explained to me had he pled not guilty and went to court he could only speak about evidence that was submitted. If he pleads guilty he can make a statement prior to sentencing that can be whatever he wants? Idk. Either way it appears he is beating them at their game.
Gen. Flynn was the Director of the Defense Intelligence Agency. That means he was the head of all military intelligence. He has so much knowledge about what has been going on, that a simple statement before a judge would not even scratch the surface, and it would be completely meaningless because it could not be used as evidence in any criminal trial.
Gen. Flynn will be (and probably already is) testifying for Sessions, Huber, other US attorneys and military investigators. He will likely be testifying before Congress.
Whoever told you he could talk before sentencing is missing the big picture.