Here is the memo.
It lists ten categories of priorities. Only two have anything to do with fraudulently obtaining naturalization. The rest apply to legally naturalized citizens. Here is the first one.
Cases against individuals who pose a potential danger to national security, including those with a nexus to terrorism, espionage, or the unlawful export from the United States of sensitive goods, technology, or information raising national security concerns;
People like Mamdani can fall into this category. I can. Arab Americans who spoke against Israel can fall into this. Chinese Americans who have connections to China can. I may be wrong about this, but these cases will be tried in civil courts, which means naturalized citizens don't have to be convicted of any of the offenses in criminal courts first.
This just one of ten. Also the closing paragraph makes it clear that these are just "priorities," not an exhaustive list: "these categories do not limit the Civil Division from pursuing any particular case."