Again, this communication from the FBI is specifically targeted at victims of a particular scam. The victims have already lost more than what the FBI could hypothetically maybe do to them.
Unless we live in Scooby Doo-verse and the FBI agent removes his mask to reveal that he was the IRS all along and this was all a ploy to get people to pay their taxes.
I think the real fear is that you never know what the FBI really wants. So unless you feel confident that you've covered all of the possible angles that an intelligence agency with 10000x the information you have could only be seeking exactly what they claim to be seeking, then sure, answering their call is fine.
But the general rule is to never ever talk with them unless you have a lawyer present. You have no clue whether you're the target of an investigation. You have no clue whether the FBI knows some percentage of their targets were victims of this scam, and are using it to get an in with them (you). You just don't know anything going on behind the scenes.
Unless we live in Scooby Doo-verse and the FBI agent removes his mask to reveal that he was the IRS all along and this was all a ploy to get people to pay their taxes.