Crossplay and cross-save are very different things.
Crossplay relies upon actual data being transferred between the two networks, and therefore requires the platform holders' mutual consent.
This is in contrast to say, how the BGS allows interactions between platforms, such as via fleet carriers, where two players chatting on discord on PC and XBox can arrange for one to fly the other to a destination system on their carrier in real time but all the interactions on each side are directly with Frontier's server.
Cross-save is entirely on the game server side, like the BGS. All the players' progression data is stored on Frontier, Bungie, Sega, etc.'s servers, and so it can be enabled as long as you set up your game's servers to be able to accept logins on the same account from any platform. Any game which requires you to register an account with the publisher's servers where your progression is stored, rather than it saving locally on your respective system is capable of this.
In the case of the brief period it worked in ED, it was clearly not set up completely, since it seemed to be just recognising you as logging in to your linked Frontier account as "an XBox user" on PC, hence PC not displaying your cmdr name correctly, presumably due to the backend code not being there (not having been y'know, actually written yet), on the account server for translating that, but the character data aside from the name was persistent since presumably the file format on Frontier's account server is exactly the same for any cmdr on any platform.