This is a perfectly normal and unexceptional application of the law governing writs of mandamus.
Fokker is not the same circumstance as the Flynn circumstances, and is as irrelevant to Flynn as the Stevens case was to Flynn.
Powell has spent a lot of time making political statements about the case, and apparently she has succeeded in making many people believe her political statements were legal arguments.
Powell’s ongoing public political statements about the case have been stupid from Flynn’s pov, but they have also been despicable because they have misled many, many people about what the case ia actually about.
The Court had several options for directing Judge Sullivan to follow the law and the Constitution which he is ignoring. They chose to do nothing and used mandamus as the excuse. A political decision, no doubt.