Deleting doesn't really delete, if you have federal law enforcement tools available. I guess the ATF hoped to be able to recover any deleted data by seizing the physical computers.
The only absolutely certain way to delete sensitive data is to get a new disk drive, copy over everything you didn't want to delete, and then thoroughly physically destroy the old disk.
Anybody knowing he's about to get law enforcement attention should backup, restore the non-sensitive files to a new disk, and leave the old disk with your lawyer (to avoid "obstruction of justice" charges over "destroying evidence"). LEOs are much more hesitant about smashing down lawyers doors and seizing privileged information.
Hey, we had a pipe burst and my computers got soaked and fried, so we threw them away and bought new ones. Prove it didn`t happen! Especially if a section of pipe did get replaced and there is water damage in that area.
They will rarely execute a search warrant on a lawyer's office, but they will serve the lawyer with a subpoena.