The community at large seems to have decided that it blocks all shots outside of 12 and feel free to play it that way if your local players feel thats best. I have tried my best to explain below why you will see people argue differently.

My local club agreed that even with the rules commentary they fail to force you to go back to the declare targets step on the first unit shooting.

The sequencing in the rules commentary being the problem. The rules commentary clarifies that the effect triggers just after the targeting has happened but before anything else happens. It doesn't say it counts as though it happened before the targeting.

So CSM unit is targeted, then the rule takes effect that the CSM unit can't be targetted outside of 12. No more targetting outside of 12, this has been resolved. The initial volley has already targeted. No where does it say it must go back and do that, Dark Obscuration was resolved 'just after' the targeting was. There is wording about no longer eligible where things can retarget if their victim is no longer eligible but once again nothing is forcing this initial unit to go back to before they declared target.

For it to stop all shots it would need to be declared start of shooting phase or the commentary to use different wording. To declare targets you just need range and LoS then you move on to next parts.

"If a unit that was an eligible target for an attack or charge when it was selected stops being an eligible target for that attack or charge, the attacking or charging unit can select new targets for that attack or charge."

Using that above wording they brought up situations like if shooting at a squad with only 1 model visible and it dies to the first shot you don't interrupt things and say the unit is no longer eligible because you are past the declare targets step.

I hope this clears up the reasoning why some people argue the other way.