Since they employed fraud to obtain citizenship, I would think their citizenship could be rescinded. If not we need a law that makes it possible that would include any that occurred prior to enactment as law.
Naturalized citizens can and have been ‘denaturalized’ for concealing or lying about their past, as well as for joining subversive groups, and even for refusing to testify before Congress. Stripping citizenship should be frequently employed these days.
By law, you are correct. Unfortunately, we all know how immigration laws are disregarded by the present administration.
All naturalized citizens can have their US citizenship revoked.