>I took a second look at ad blockers on the app store, and many report that they collect various bits of data.

Because the "app privacy" disclosures that apple only contains broad categories about what data the app can possibly collect. If the app collects analytics in the UI itself (ie. the part where you select filters or whatever), it has to say the app collects analytics. It's not possible to say "we only collect analytics on your usage of the app, not what your browsing history is".

>Are you saying that there's a special content blocker component to all of these that can't collect data because they're isolated by iOS?

Yes.