Sony already technically allows this.
The previous arrangement was, simplifying the details of course, something along the lines of ...
If you are developing for the PS ecosystem, and use their services and servers, you are locked into PSN connections only - only PSN connections can connect to PSN servers (for the most part). If you don't use their services and servers, and build your own infrastructure, you can allow connections from anywhere, regardless of its origin. This allowed open PSN-PC play on games that setup their own matchmaking and servers. And, in theory at least - because Microsoft blocked these from ever happening on their end - connections from XB to the privately run servers of a PSN game.
Obviously there are more security requirements and standards, but for the most part, if you run your own matchmaking and servers, Sony doesn't care where the player connection comes from.
If you were developing for the XB ecosystem, no matter whether you use their services and servers or your own services and servers - all servers on XB must be XB servers - and just like PSN, only XB connections can connect to XB servers. However, you could run an XB server on PC and allow crossplay between XB-PC. The problem is that you can't run a XB server on PSN or allow connections from PSN to an XB server.
Microsoft actively prevented any non-XB ecosystem connections from ever connecting to any server within their XB ecosystem, even if you ran your own matchmaking and servers. And that prevented anything but specific XB-PC crossplay, and only when PC is also running XB ecosystem servers.
Without knowing more details, I have to assume they are finally moving to Sony's requirements - if you run your own servers, it doesn't matter where the player connection comes from. That said, their line about "open invitation for other networks to participate" makes me think they've setup their own conditions, security requirements and standards for what a non-XB server must have and developers will now have to have servers that meet both Microsoft's and Sony's third party multiplayer requirements in order to allow play between the two ecosystems.
Which is fine, as long as it's "meet or exceed", and not a demand for Sony to conform directly to whatever specific requirements Microsoft has deemed appropriate.
Also, for what's it worth, there have been games that have tried XB-PS crossplay before - I've worked on one myself - and Sony was always fine, given their third party multiplayer standards were met, while Microsoft would flat out tell us a hard "No". So while we haven't had any XB-PS crossplay titles actually launch, there have been attempts in the past and Sony has shown no indication - to developers - that they have an issue with it. Microsoft has. Until now at least.