They’re not done yet. Just announcing (and the verge reporting on it).
Their announcement (here) is quite forceful though, interestingly. The article described it as a manifesto.
See also a recent post here about their survey on integrating activity pub: https://lemmy.ml/post/14734757
It’s talking about new networks not already established ones.
New? Old? Don’t think it really matters in this case
Of course it matters. A new network will have to start from scratch and with the “nobody is there, so I won’t join”-mentality, potential growth is severely limited. By implementing federation, you will instantly have a base community to build on.