> We are aware of this matter, but the specific Terms of Service violations are ones we cannot discuss further due to legal considerations
I think the most likely scenario has to do with cp. IANAL, but I think in many jurisdictions cp has strict liability, meaning that if you are in possession, knowingly or not, you are criminally liable. If the author was hosting or somehow tied in to cp, the authorities would likely force Google to take it down and hand it over for investigation. The case is probably still being investigated so no action has been taken, but if Google hands over the archive, it would likely be liable for dissemination of cp.
This seems to be a lot more reasonable explanation to the secrecy than the standard "Google is evil" argument. The main thing that makes me doubt this explanation is the authors vocal outcry. If he were involved in some nefarious activity, I would imagine he would just disappear into the shadows if he suspected his acts were close to being uncovered