The L5 uses automotive-grade RAM (Micron MT53E768M32D4DT-053 WT:E), which has a unit price of $21.75, whereas standard RAM should be about half that price. I suspect Purism selected automotive-grade RAM because most of NXP customers are automakers, and it was one of the models of RAM that NXP says that it has tested and Purism didn’t want to test other RAM.
The schematics have a floating comment with various models of RAM and Flash memory that I suspect Purism was also considering. One of them was Micron MT53B1024M32D4NQ-062 WT:C TR 4GB RAM, whose unit price is $57.72, which is very expensive. You can get 4GB LPDDR4 SDRAM that costs $15, but Purism probably didn’t want to take the risk of using RAM that hadn’t been tested and recommended by NXP.