I don’t know a lot about how states conduct cyber espionage against one another, but it does feel a bit off to be told that this was the work of a nation state with zero proof as to why.
After Jamal Khashoggi was assassinated, for example, it turned out audio of the entire murder had been recorded through surveillance. (I'm still not sure we ever found out how exactly it was recorded?) So there were lots of announcements that certain governments "knew" or "suspected" it was Saudi Arabia with little or no substance to back up their claims until the existence of the recording was later leaked.
I think there was similar discussion after MH17 when western governments "knew" Russia-backed rebels had shot down the plane and only later did we learn there was a recording of a conversation about what happened.
I guess even after Snowden governments like to do their best to pretend they don't have the entire world under surveillance at all times.
I would say it’s also quite likely that the US knows who did this or has known who has been doing it for a while. It’s highly unlikely that this press release wasn’t coordinated by the intelligence agencies to play into some advantageous scenario they’ve drawn up.