Sure. Essentially the courts are affirming that the system is “first to file” and not “first to invent”. You cannot quietly invent something and then claim ownership after someone else patents the same idea. If prior art could be private, then the system would still be de facto “first to invent”.
How well known something must be is obviously going to be subject to interpretation. If you tell your co-worker at the bar, that’s probably not sufficient. If you publish on your blog that no one knows about, that’s probably also not sufficient. If you publish in a journal, that probably is more than sufficient. If you cannot reasonably claim that the other inventor should have been able to find your prior art, then the courts will rule against you.
How well known something must be is obviously going to be subject to interpretation. If you tell your co-worker at the bar, that’s probably not sufficient. If you publish on your blog that no one knows about, that’s probably also not sufficient. If you publish in a journal, that probably is more than sufficient. If you cannot reasonably claim that the other inventor should have been able to find your prior art, then the courts will rule against you.