> How would US attribute who's sub it was?

The US has quite a.lot of ASW resources, can identify subs, and you can't do that attack without providing a position fix.

Also, the US has other intelligence sources.

Also, current political situation helps narrow likely attackers.

Also, and perhaps most importantly, not a lot of people have hypersonic weapons and subs to deliver them near the US. Flashy unique capabilities are great in many ways, but they are absolute crap for obscuring responsibility.

The US pretty much knows the location of the Russian missiles subs most of the the time. It would be a bit crazy to gamble on being sure the one you launch from was the one that was missed. Also most of the Russian boomer fleet is in dock most of the time.