I presume that the SAR investigators were able to rule out the possibility of coded tags, for lack of that unique identifying number? (And why doesn't an aviation ULB broadcast the same unique information?)
If the audio was modulated with a code then that would have been very obvious when they analysed the signals.
As for uniquely coding aviation ULBs - there's not a lot of point since, unlike with tagged animals, in any given ocean in any given 30-day period it's hardly likely that there will be more than one pair of ULBs down there!