Why would the police even inform you that flock was used?

Why? Because the prosecutor doesn't want all their work to go to waste because they didn't disclose Brady evidence. Even if they successfully argue that Officer Flock's reporting isn't exculpatory, they still have to do extra work to respond to a Brady motion for a case that already got a conviction.

In cases of parallel construction of evidence, is the prosecution still obligated to disclose everything to the defense in discovery? Is the tree from which the Fruit is picked only obscured from the jury, or from the defense as well?

Any evidence the prosecutor uses that is potentially exculpatory must be disclosed to the defendant. So your questions raise why they wouldn't: they think the defense can't find out. And it's arguably likely that they won't. But if they do, it will not only invalidate that single case, it could potentially trigger retrials of hundreds of cases because now a bunch of people know that Officer Flock is on the Brady list.

And one final thing about this... the prosecutor (who has probably said some variation of "ignorance of the law is not an excuse" to try to get a conviction) can't claim ignorance as a defense to a Brady violation. Failing to disclose, whether they knew or not, is a Brady violation.

Hah, as an ex-Flock employee, their transparency report is utter garbage. "Agencies using Flock." In my immediate vicinity, three County Sheriffs use it, but they are not listed. Several city PDs in my county also use it, but they are not listed.

Discovery applies even in cases where the state is prosecuting you.