No, your fingers and palm grab and touch the back of the iPhone 99% of the time. The matte glass is slippery, the smooth glass is grippy.Unless you balance your phone's back on your fingertips, the sides are what dictate how slippery the iPhone feels. The back of the phone rarely touches the palm of my hand.
As you just said, the flat sides provide a better grip, and you don't have to grip the flat sides as tightly which improves ergonomics.
The back glass being smooth and grippy is far far better to grip and not drop the device. You erroneously think that the device rests in the palm. No, that’s not how phones are held you place fingers across the back, in particular, your index finger for grip and stabilization.
Also, and highly importantly, when you pull your phone out of your pocket all of your fingers grab the back of the device to pull it up and out, and smooth grippy glass allows those fingers to grip it much much much better than the slippery matte glass.
Flat sides do offer better grip only because they dig into your hands.
And no, that is factually untrue that it ‘improves ergonomics because you don’t have to grip it as tightly’. That’s one of the most false and hilarious statements I’ve ever read on here. The square edges dig into a user’s hands and they are pathetically bad for ergonomics. There is no ‘lighter’ grip lmao. It’s horrible ergonomics and feels terrible held in your hands.
Go purchase a squared off steering wheel, you won’t have to grip it as tightly so it will be better ergonomics. LOL. This is sarcasm by the way.