I was going to make a snarky comment, but I realized it's possible you just don't know how cookies, 3rd party or otherwise, work.

3rd party cookies are the original way advertising companies spied on users, and the only way the vast majority of companies can spy on you. What they do is make is so that when you request a resource you can attach cookies to the request that are not from the domain making the request, so that if you have multiple unrelated sites requesting https://advertiser.com/resource they will share the same cookies and allow the operator of advertiser.com to uniquely identify the same user regardless of the site actually being visited. Wikipedia has a big page on this specific technical concept: https://en.wikipedia.org/wiki/Third-party_cookies

This is what a "3rd party cookie" is in the context of blocking "3rd party cookies", and this is what more or less every browser other than chrome now does (except maybe edge?). Safari has had blocking 3rd party cookies be the default behavior since the very first betas. It was the first browser to do this by years, to this day I'm not sure why Firefox didn't immediately follow suit, but that's something best answered by someone from Mozilla of the era.

The article you're pointing to is discussing a further hardening of the restrictions, ITP, and origin based cookie segregation. These are all increases in the degree to which cookies are blocked, and these are necessary specifically to deal with privacy invasive tracking that companies like google and Facebook are able to do.

For the overwhelming majority of tracking networks simply blocking 3rd party cookies is sufficient. But over the last decade or so companies like Google have aggressively introduced new mechanisms to promote their 3rd party cookies into the sets that can be shared. Google has been very aggressive in this by working extremely hard to get as many webdevs as possible to add spyware to their pages to get "metrics".

There are numerous steps they take - redirect loops were in vogue for a while, i'm not sure what they're doing now - basically trying to either link a cookie from the domain embedding the tracking code to a cookie on the advertisers domain, or promote the "3rd party" domain into being part of the primary site. Defeating that requires cookie segregation (so that every site you visit has a different cookie vault for every different origin it contacts), and things like "tracking protection" which tries to detect sites that are being pinged from many different origins (implying they're for tracking rather than site content) and severely curtailing any cookies for those origins.

The post you linked to is talking about that, and it sounds like an end game step which is that loads to any resource from a different origin gets no cookie state at all, which historically didn't seem possible due to many weird ways sites managed account login and the like but maybe things have changed since then.

This world of cookie segregation and tracking prevention is _significantly_ stricter and more powerful protection, and is far beyond the "3rd party cookie blocking" that google is still delaying to this day, and has been something safari and Firefox have been doing for years, but it came after, and in response to, companies like google trying to circumvent the privacy provided by 3rd party cookie blocking.

This is also why google is now talking about blocking 3rd party cookies - they've spent more than a decade come up with ways to track people in spite of 3rd party cookie blocking, and they have no implemented any of the privacy protections Firefox and safari have been shipping for years (nor is chrome likely to implement anything of the kind). Google (and FB, etc) are in a position where they can do this, but smaller advertising networks can't (google has tracking code on almost every page as part of their "we'll provide you with analytics/metrics" scam).