Flexi-cuffs usually have a wire running through them which makes it far more difficult to cut through or pop off, unlike zip-ties which are generally just plastic except for the tiny metal blade inside the head that locks them.
Flexicuffs intended for LE use are significantly more expensive than simple zip ties, and I believe are marked with a lot # which would make it relatively easy to confirm/deny if they were sold to an agency active at the capital on 1/6.
Yeah, we used them in the prison system. We also used cuffs, but only we Sergeants carried them, and only had one set issued to us each shift. Once we put them on someone, and they were taken to the box, if we needed to cuff anyone else before we got our cuffs back, we had to use flex-cuffs. The fact that the guy in the picture is walking around with them in his hands, makes me think he did find them laying on the floor. Why would he bring them out of a backpack or pocket, and carry them around, if there wasn't even anyone in the chamber to put them on? If that's what the Feds are claiming his plan was...it's pretty lame.