You're correct, that his throws aren't exactly following textbook mechanics.
His throws these days are more of a sidearm motion, which generally isn't ideal for a quarterback. His throwing motion in high school looked more like the classical overhand motion.
This isn't to say that a sidearm throwing QB can't be good, since Bernie Kosar was pretty successful, but the rest of his mechanics were solid.
Look at Arch Manning's throws, especially his shoulders, hips, and feet when he throws. A lot of times, he's not syncing up properly, and he's overcompensating for it with just his arm.
I'm no quarterbacks coach, but I can see sound bodily mechanics, or the lack thereof, after having trained in various martial arts for 40 years, and taught regularly for the last 20.
The mechanics for delivering a solid punch have a lot of overlap with throwing a ball, where you use your lower body to drive forward with power, rotating the hips to generate more strength, and synchronizing the lower and upper body together, making sure that you're engaging those core muscles.
Someone can watch a Mike Tyson fight, and see that he doesn't use much pure arm strength in those vicious punches that KO'ed a lot of guys. His best hooks and uppercuts were actually very compact punches, and he'd use incredible leg drive and hip rotation to send a lot of those guys crashing to the canvas.
The way Manning is throwing the ball, is like someone trying to throw a hard punch using mostly his arm, and not optimally using the lower body. I'm surprised that Steve Sarkisian hasn't worked with him one on one, given how he brought up those 1st round picks from Alabama.