If the US really regarded cyberattacks as an act of war, it would be actively shooting missiles and bullets at China right now. The US regards cyberattacks as acts of war when convenient, nothing more.
"Act of war" is a legal and diplomatic nicety that, like essentially all international law, you talk about when it happens to align with your realpolitik.
It doesn't mean you immediately launch the nukes whenever someone arrests one of your diplomats.
Nuclear powers can't make full-scale war on each other. If they ever do, the death toll would make WW2 look like a skirmish. So any attacks are at the edges (respond in kind, or proportionately, rather than escalate to a shooting war).
So the US regards cyberattacks as acts of war if it comes from a non-nuclear country, and tomfoolery from a nuclear one?
I don't disagree with your point, simply saying that IMO the US doesn't take cyberattacks seriously precisely because it leads down a dark path with China (and probably Russia too if we're being honest).
It doesn't mean they don't take them seriously, it means that responding with missiles isn't an option if the other guy has the ability to massively retaliate. Other options, like sanctions or retaliating in kind, plus shoring up defenses, are a better bet. Even retaliating in kind is problematic if the result is that the Internet becomes almost unusable.
All during the Cold War the USA and USSR engaged in various kinds of low-level sabotage against each other. Fortunately for us all, it didn't escalate out of control.