Prop money is a tricky proposition. To do it legally it has to be immediately clear the money isn’t real. I found that particular thread with a quick reddit search for prop money brands. Apparently it’s kind of a go to, but all of the threads are about how obviously fake the bills are in person.
So it’s probably fine. Friend of mine used to use bills with the Golden Girls in place of the usual faces, and the more realistic ones I’ve seen in person the inks are obviously the wrong color even though they show up close enough on camera, and the paper is completely wrong.
You’d have to be really not paying attention not to catch it. Basically most of this stuff won’t pass face to face, or with too direct of a shot. Clean shots of them mix in actual money where neccisary. You also have glued blocks/boxes for money that doesn’t need to be spread or handled, blank green sheets for filler, one sided bills, all for different situations. With double sided accurate stuff being just for close, direct shots where you need more than you could cover with actual cash.
The companies that have seen seizures got too close with their most realistic bills from what I understand. And most of them got dinged because their props were actually being passed.
For that reason I tend to think deliberate counterfeits have to be cheaper. There’s a lot of liability and covering your ass in selling legitimate prop money. Good counterfeits might cost more to print. But you’re probably running off bigger batches and you’ve got fewer lawyers and insurance companies involved.