Reading between the lines, it is the same attack that hit google way back when. Google disclosed it, and disclosed that 20 other companies were affected, and none of the other companies came forward.

Quite plausible, but what evidence is there to support that?

I don't have any other than there were 20 other companies who were not named at the time, Google was hacked by chinese military, now Yahoo claims china.

Deduction on my part, but no further evidence than that.

Sure, but China has also (allegedly) been responsible for many other breaches unrelated to the Google incident.

Because the Russian hacker blame game is passe.