My guess is that this operation was already being planned years ago and it was just a matter of timing to happen after ChipMixer went down.
The beginning of investigations related to CM started in 2021, as mentioned here
https://www.justice.gov/opa/press-release/file/1574581/downloadBetween October 20, 2021, and October 22, 2021, an FBI agent accessed the
ChipMixer website located at the domain chipmixer.com.
And if we noticed that since the middle of last year, DDOS attacks were abnormal for the site, and therefore they may have been able to access some data and continued to make the site work throughout this period before they accessed NGUYEN aka CM.
So the case is several years old and they may have accessed the data before March.
What will prove this theory is if more dark web sites were closed during the current (and next year) more than the average in the past years, then there is certainly important data they obtained from CM.