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.