If you believed single n-back was going to definitively beat the active control, then you wouldn't pay for a passive control. I buy that. But now that it hasn't, it's worth adding a passive control.

Some apparently randomly chosen training task (vocabulary and trivia memorization) exhibited just as much generalized training as single n-back. In your interpretation, neither had any generalized benefit, then - the improvement is just due to normal ~9yr old child development over the timespan.

I do recall hearing some credible evidence that dual n-back (whatever configuration was in some older Jaeggi study) gave a boost to "fluid intelligence". (thus the interest in the topic). But now I'm given to mistrust Jaeggi more than I would the average influential researcher.