Don’t they usually have “styles” of intrusion? I’m certain I’ve read in the past where intelligence agencies could differentiate between Russian backed styles of hacking and Chinese backed styles of hacking. I’m sure Americans have their own style of hacking as well.

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.