Has Flynn ever stated that he plead guilty under duress? It would seem to me, a non-lawyer, that anything said in court under duress is cause for dismissal.
Sullivan then asked Prosecutor van Grack whether Special Counsel Mueller's Office had considered charging Flynn with treason. That would easily induce stress for almost anyone. Any decision Flynn would have made in that context would inherently have to be considered "under duress."
Although Sullivan backtracked from his statement, the damage had already been done. One could easily argue Sullivan should have been forced to recuse and that the hearing should be redone with a different judge. The way it was left, however, did put another arrow into Powell's quiver to seek Sullivan's recusal and have the case against Flynn vacated, the latter of which was so far done by the DOJ.