The Rogue ones are competition bumpers and a little thinner. Mine come in at 3.5 inch on the 25 kg. So a little thicker. I’m not suggesting it’s a grip exercise. Just that that the thicker the bar/plate the more it works grip.
As for never hearing of the two hand plate curl This came from powerlifting guys. As in world and national championship level powerlifting guys. That’s where it was learnt. It not meant to be a pure strength exercise. It’s used at the very end of a 90 minute workout when the muscles are already pretty fatigued and well used for high reps.
Bear in mind too, it’s a filler, as I mentioned: lockdown, gyms closed. Only equipment is barbell and plates, no bench. Plenty of other barbell exercises thrown in, but this was programmed in to for some variety and mainly for some hypertrophy and assistance to the big lifts.