Sorry, you're wrong. Flynn stood up in court and detailed the actions he agreed to with the DOJ.
Yes-- the DOJ said this is what had happened. He agreed.
He knew that he had not taken those actions (if he is to be believed today) and yet he stated under oath that he had in fact done what he was accused of. To knowingly state something in court under oath that you know to be false is the definition of perjury.
Well, on that vein, Flynn never saw the transcripts of the conversation until recently. Nowhere in it does it discuss sanctions. So maybe he would not have pled guilty had seen the actual evidence.
Or maybe the DOJ would have been unable to prosecute and thus offer a plea deal.
What you are saying is Flynn articulated details of crimes he didn't commit and he did not know that he had not done them.
Part of the crime for which Flynn was charged requires the statement made to be "material" to the government function. The DOJ said it was "material" and Flynn accepted their argument. He pled guilty. He had no way to dispute it.
Subsequent evidence released shows the information was not "material" to anything. Thus, the charges were dropped.