I haven't done any analisys, but here is my take on this.
There is no doubt that Nikon knows what they are talking about when it comes to lenses and lens care. However, in this case I suspect they are covering their butt. Unfortunately manufacturers are driven to do that more and more because every once in a while someone does something stupid, perhaps deliberately, and some sleazy lawyer hypes it as uncaring giant corporation screwing the little guy and gets a jury to fall for it.
Your breath is saturated with water vapor, which condenses on anything colder. In the winter when the outside air is sufficiently cold, it even condenses just in the air and causes fog. This condensate, whether the fog in the air or the thin layer of microscopic drop on a hard surface, is essentially distilled water.
Now distilled doesn't mean pure, but it will be pretty clean. Only the stuff already in the breath can condense, so we have to consider what exactly is in breath. Most of the breath will be nitrogen, oxygen, and carbon dioxide. Those will remain gasses at normal human temperatures, so what we really want to know is what else is in this mixture in partial-pressure form that has a lower saturation level at lower temperature. That's going to be overwhelmingly water. I'd like to know what acids Nikon thinks are in breath in partial-pressure form. The body has some hydrochloric acid, but that is in the stomach and not in your lungs. The most likely culprit is carbonic acid, which wouldn't actually be in the breath, but could be formed by the condensed water particles dissolving some of the CO2.
Maybe Nikon is referring to this weak carbonic acid, but my guess is that they don't want to get into a case where they said breathing on the lens was OK and then somebody claims damage when they actually got spit on it.
I have breathed on camera lenses many times over many years and have so far not noticed any ill effects. And yes, these were overwhelmingly Nikon lenses.