Ya there is more than 0mm for sure! But the rust could be hanging the pads up in the future.
Some shops suck too, I got new tires on a rig I had just bought.....one of the guys came to me and told me my brakes were super bad, and I need to replace them...I was laughing inside, but played dumb, it comes naturally....so oh wow really? Can you show me...um sure he says, so we go out and take a look, he points and says they look thin! I say ok, a 5/16 is thin, he says ya, the looked brand new still had all of the paint on them. I said ok, so are you going to warranty them out? He said what? Ya warranty them out, you guys install them less than 5000 miles ago! Haha! He said oh, well I guess they aren’t that bad! I couldn’t believe they were that bad, I had the paperwork from when and how many miles where on it, and they looked brand new! I was pissed!