Yes, but the proximity is such that the soundwaves themselves could conceivably vibrate the case enough to do damage if it is in a partially enclosed space and facing out (because that way it gets pressure from only one side, essentially a reverse bass speaker).
It's not a bad hypothesis, but would Apple have withheld that info if they were already advising people of the danger of direct mounting? They could just advise people to keep their phones inside a padded box or something of the sort.