It's hard to say given that this is brand new, but tighter integration with Godot is a boon. They're offering on-demand server access, with instances running Godot for access to the high-level network API. Steam is built around either P2P or dedicated servers, not on-demand cloud servers like this. In theory you might be able to set something like this up with the Steam API, but you'd need a devops team to set up and maintain it. They're offering API support+cloud from the same company, which if they're any good is potentially valuable to customers. Even if you just use their code, that's a head start.

Looks like they also support P2P, but it seems like their big offering is their server architecture.

Who knows how well they can deliver on those promises. A more concrete offering is that if you use the Steam API, you're locked to Steam, while in theory this works wherever Godot works.