And how are they supposed to get these statistics?

As far as I know, they make these estimates on the basis of how many sanctions and repayments the Job Center imposes. But since it's not easy at all to get by these people and the effort isn't worth it, there is no way to say how many people are doing it.

But if you try to convince people to think smart(er) about the problem, it doesn't help to deny the reality they are actually living in. It would make more sense to show how it's the employer who makes profit from tax evasion.