They were claiming that specific manufacturers had not used their allotted cap, so they refused to raise the cap of other manufacturers that had reached their cap.

DEA isn't the issue here.

The Justice Department brought the lawsuit against Purdue Pharma, and distributors are scared.

As far as I could tell reading that DEA memo, they were being really sneaky with what they said. None of the pill manufacturers had hit their allotment, but they didn't say anything about the raw ingredient manufacturers which are under a different quota. And most of the shortages I've seen in the FDA shortages database were listed as 'shortage of active ingredient'. (Or 'demand increase', which isn't specific enough to know that it's not literally the same thing.)