Playwright is shipping patched browsers. They take the open source version of the browser and patch in e.g. CDP support or other things that make automation "better. Playwright does not work with a "normal" Safari for example.
It's real Chromium, not emulating a Chromium browser. "Old" Headless was merely pretending to be a Chromium browser, the "New" Headless is a Chromium browser. "Old" Headless requires a parallel/duplicate implementation of features, which leads to subtle behavior differences or infeasability to support certain features e.g. extensions proper.
We regularly get feature requests for Headless to provide a field or property that can be polled by JS frameworks to detect if Headless is active e.g. windows.isBot.
Well, Headless is open source, which means anybody could build a Headless version with such a property set to "I am a human, trust me!" and employ such a modified binary ... ;-)
PM working on Headless here. Masking bots is not the reason why the new Headless mode was created. The goal is to provide an headless browser that can be used in web tests. The original Headless is essentially a separate browser implemented in parallel to "proper" Chromium. That results in all sorts of subtle reproducibility problems for developers using Headless for their tests.
I used it in the past and I think it is a good framework to discover uncertainty and make it more visible, because it makes you talk about optimistic and pessimistic cases.
The estimates are also good enough to come up with a draft schedule.