Thats why I presume its asking about previous engagements, if they catch someone they suspect of espionage, dig into their background and find proof of previous activity they have a clear fraud charge without having to prove their suspicions about current activities.

There's often also some arbitrage on standard of proof or statutes of limitation or jurisdiction.

Maybe to deport you for espionage requires a jury trial, but to revoke status for misleading answers on an immigration form is administrative and so is deportation for lack of status.

I seem to recall some extraordinary cases where untruthful answers on immigration forms were used to justify denaturalization.

Proving you worked for a spy agency is far easier than proving you did spying in actuality. Assuming you didn't get caught in the act.