Seems like really vague speculation. Any evidence this is not pseudo-engineering nonsense?
Typical smart phone / tablet type products use a stamped metal frame ($) to stiffen the product and support the display. A few phones use die-cast magnesium frames ($$). A CNC'ed pocket ($$$$) is a significant cost adder for this product (larger starting billet, longer cycle time, and thousands of CNC machines required) -- so this choice was clearly made for a reason.
Additionally, the fact that the battery is surrounded on five sides by a machined pocket cuts into battery volume -- most products just rely on an air gap (0.5-1mm) between the battery pouch and the next nearest component that could poke it during assembly and usage. Take a look at some other product teardowns on iFixit, and you'll see this (example from iPhone 7: https://d3nevzfk7ii3be.cloudfront.net/igi/fJI2bhqYXg4CMNPi.h...)
They added significant cost to the product and sacrificed a little battery capacity to have that pocket. I hypothesize that it was for battery safety given the aggressive design.
Compared to cast or machined parts, you get a 20% higher strength-to-weight ratio from the forging. The cycle time is decent I think, perhaps 3 per second with a 0.06 second contact time in the dies.
If my assumption is correct, though, I find it strange that article footnote seems to contradict this by essentially saying the engineers didn't give enough space around the 'front' and 'back' of the battery. Did they protect the PCB and battery from each other, and fail to protect the battery from the front and back of the phone?