Strategic ambivalence
Proportionate response only makes sense in a fairly narrow range of scenarios. The biggest problem with proportionate response is that it allows the adversary to control your level of response. It is quite easy to create a list of ways that one country can mess with another short of a military attack. The entire point of the term casus belli is that the justification for war need not be "proportionate". So, some minor member of nobility gets assassinated by a rebel faction & now the Austro-Hungarian Empier and Russia are at war. Or some French Diplomat asks a Prussian diplomat if they will carry out their promise for territorial accession & now the French are at war with the Prussians.
What the general has stated is that "mere" attacks on the IT infrastructure will be treated just like anything else--a potential justification to strike back at a time and in a way that is chosen by NATO.
Of course, attribution is a major problem. But spycraft has had to deal with such issues for millennia. They make the determination as best they can, and the base their responses on the reliability estimates of these determinations as well.
Biting the hand that feeds IT
