The page states that the LTE models were manufactured using a process called co-molding. It’s a high temperature process involving the injection of plastic into precisely milled channels within the aluminum case.

At the end of the manufacturing process, Apple allowed for no more than 400 microns of deviation across the length of any side. The company went on to say that the new straight-edged design of the iPad Pro may make bends “more visible only from certain viewing angles,” and won’t affect the tablet’s performance. For anyone who feels that their iPad Pro is bent beyond the 400 micron point, the option for a warranty claim is still possible. Apple does state at the end of its new support page addition that it covers damage due to defects in materials or workmanship. (Source: Apple)

Apple Explains iPad Pro Manufacturing Process In Response To Bent Units - 21