While I agree with your basic premise, everybody gets things wrong sometimes, I still feel this was needles dramatization by Schneier when I read it back then.

Somebody with his experience should know better than to throw guesswork attribution haphazardly around like that (at least not without a very big disclaimer), especially in a climate that was, and still is, pretty hawkish on "cyberwar turning hot".

Because it's exactly his experience and prestige which gives the uninformed the impression that his "Russia/China" attribution was based on something solid, and not just mere guesswork without any basis on evidence at all.