This lawsuit is idiotic. They're claiming Google is doing something wrong by tracking users from websites using ad tech when someone uses their browsers private browsing mode.
What Google is doing is wrong. That's literally what the lawsuit is about.
DNT is a header, but it's a preference and not legally binding.
The only thing Google did that was "wrong" is that incognito gave uses the impression that they weren't being tracked externally, when in reality the only thing it does is not save your history and start with no cookies.