Flipping this around, do you think Google's legal team, which is probably equally qualified and vastly more well paid (and with a lot more to lose if they're wrong), would advise employees to do this if they hadn't considered such trivialities?
Well, I would expect Google's legal time to advise employees not to do anything illegal, yet here we are.
Depends on what their risk-reward appetite is. Competent people will make surprising decisions under pressure.
That's the same for DoJ. In fact, for them it's all wins and no loses. In the worst case their argument just doesn't stick.
There is already a lot of precedent around policies related to spoliation which is why it would be a surprise to me if Google's legal team actually did advise employees to do this as a blanket policy. It seems more likely that they said they could do it for things that are not potentially relevant to litigation and the execs took it as far as they wanted to go.