McCain just claims there was nothing in there that would have been
"disqualifying". Incredibly wealthy people not paying much or any taxes is not against the law, nor is it very objectionable to Republican congressmen. After all, they are the ones who created the loopholes to allow them to do it in the first place. Furthermore, the information would be safe from prying eyes because vice presidents don't need to divulge any of their tax returns. McCain readily admits the reason he didn't select Romney was largely due to his wealth. McCain even refused to reply to the question of whether or not Romney paid no taxes.
But I agree it is highly unlikely that Romney did so. I think it is just so mind-numbingly low amount that it is going to be a huge embarrassment to him. Romney is not about to willingly give up ammo which would likely help assure his defeat.