I wonder if this is just a token effort by the DOJ just to ameliorate their petulant POTUS. Even then, it is extremely alarming. Just another piece of evidence of the weakness of our system that relies of good faith following of norms by those in power. Nothing really stops the president from personally directing the DOJ to extract vengeance from his political enemies.
Realistically, there are lots of good business reasons for automakers to agree to the California standard. I imagine all the current engineering has been done to meet the previous standard, and redesigning would be expensive. Maintaining separate designs is probably more trouble than it's worth. If Trump is a single term POTUS, odds are good that the EPA decision will be reversed and they'll be back to the old standard soon enough.
I don't really see any price fixing here. They are agreeing to an emissions standard, but these companies are still very much in competition with one another. It's going to be tough for the DOJ to argue that industry standards that aren't explicitly written into the law are anti competitive.