Their mismanagement of generation contracts was somewhat forced upon them and the CPUC was complicit. They were forced into long term solar and renewable contracts long before solar pricing came down. They now own 7,600 MW of generation. That isn't a trivial amount. Of course that number will come down with the closure of Diablo.
But I still say all the CCAs do is insert a middle "buyer" between Generators and IOU's Distribution. If the IOUs were better managed (which is part of CPUC's job as an oversight), then CCAs would not be required. And, in fact, CCA's are creating competition for buying electricity which could drive up the cost of the source generation. Why would a generator sell to a CCA at a lower price than they could sell to an IOU?
Your statement about following CPUC instruction is mostly true