This is clearly illegitimate, they can tell that much. They just got the use case wrong. It's for fraud, not terrorism.
How do you know?
(BTW, I'm not suggesting that you are wrong. I have no idea. But in my experience with Federal law enforcement operations related to technology, they're not typically so incompetent as to confuse a fraud ring for a more serious operation. I choose to give them the benefit of the doubt.)
I'm all for having a productive discussion, but casual exaggerations and half-truths aren't helpful. If you just don't trust LE, that's fine (and quite understandable), but that's a more honest thing to say than that you know something contradictory with absolute certainty.
[1] This one is at the top of the front page as we speak: https://news.ycombinator.com/item?id=45357693
And it’s not sealioning; I'm not making a bad-faith argument to wear you down. I’m saying something really simple: unless you know something with absolute certainty, especially about a situation that doesn’t involve you, expect to be challenged when you claim you do. We have qualifying vocabulary for this very purpose. It’s why reporters use the word “alleged” when referring to criminal defendants who haven’t been convicted. It’s a pretty straightforward principle, and there are plenty of responsible people out there who formulate their doubts with the requisite nuanced writing. If I can do it, anyone can.
And no, it’s not unreasonable to demand that someone support their unqualified claims of certainty. It will be the Secret Service’s responsibility to eventually substantiate their claims, too.
You only need this many for bulk messaging/calling. Legitimate bulk messaging/calling would be going through sip providers and SMS aggregators and/or interconnection with carriers at the kind of volumes you'd have this many sim boxes for. So it's got to be fraud/abuse of some sort. Probably selling bulk sms/calling to users that can't or don't want to use legitimate providers.