They are also part of the Chromium project, so you're welcome to check it out, inspect the source code, and build it locally.

Just because it's open doesn't mean anyone can do such thing. I'm a professional dev and I can't browse such a complex C++ codebase.

I was being flippant, but isn't that true of anything though? How do you know Linux isn't secretly reporting all your activities to Big Penguin?

You should be able to trust a popular open-source project, because you trust that there are folks who will go in and take a sticky beak at these kinds of things. This why a lot of security folks prefer open-source software over close-sourced solutions.

Surely, if Chromium were cross-correlating profiles, then it'd be on the front page of Hacker News in short order.

IME they do cross-correlate...