On the other hand, they write:
[S]enior government leaders should be held to the highest standards. The defendant’s extensive government service should have made him particularly aware of the harm caused by providing false information to the government, as well as the rules governing work performed on behalf of a foreign government.
There were three distinct areas he lied: His paid work for Russian state media, his lobbying work for Turkey (which may or may not have involved an unacceptable plan to kidnap the cleric and remove him to Turkey, bypassing the legal system)and his meetings with Russian diplomat Sergei Kisylak. The New York Times had this helpful graphic.
The Flynn sentencing memo certainly has a different tone than the memo the Government filed for George Papadopoulos, whose sentencing guideline range was also 0 to 6 months. Team Mueller asked for "some incarceration" for him and he got 14 days. (He whined all the way to the prison door,threatening to withdraw his plea, asking for delays, sending his wife to appear on Fox News to plead his case on TV).
It's not too often a U.S. general gets sentenced for committing a crime. The last one I remember is General Petraeus, who pleaded guilty to a misdemeanor count of unauthorized removal and retention of classified information and was sentenced to two years probation. He also lied about the disclosure when questioned by the DOD about it.
Mueller isn't saying exactly what Flynn did that was valuable, only that he may not be done investigating what Flynn told him.