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.