@CaliforniaCookie @AliceOlive as noctilucentcloud says... it adds more fear.
You view the compressed air spray as what it really is, a harmless can of air that makes a hissing sound.
The dog doesn't know that. For the dog it is a sudden unexpected and startling sound, it gives the dog a jump-scare.
Now think how many times you'd need a jump-scare, unexpected startle, before you started to feel quite jumpy, on edge, tense etc etc? Even though you were not physically harmed at any point.
We do not need any physical contact or pain to have an aversive experience - I usually use the analogy of being in a bank during an armed robbery - just the sight of a balaclava'd person pointing a gun at you would cause you extreme distress, fear, potentially causing all sorts of responses from you that you'd not be in control of, and possibly leaving you with lasting trauma. Even though at no point were you actually caused any physical pain nor was there any contact.
So back to the dog - they're feeling unsure, insecure, anxious, fearful.. and then something even MORE scary happens - it might stop the dog barking for a moment, which might lead you to think it has worked, but the barking/lunging/growling etc.. they're a symptom of the fear itself. That fear still remains, the corrector can hasn't done anything to change that.
There is of course a chance that the dog may then realise, for some other reason, that this person is not in fact scary. If so, great... but it is a chance. It is not a guarantee.
There is a much higher chance that you have made that fear worse to the point the dog no longer dares bark - so they may now bite without warning (dogs very frequently forget they can move away, and get 'stuck' as the scary thing approaches. People do this too!).
You may also have caused (via single event learning in some cases, just 1 aversive experience) a lasting fear of that particular noise, of the sight of a spray can, of similar noises or other events closely linked to this aversive event.
This is the problem with aversive methods.
you cannot know the damage it may do, until it is done.
Fixing things like a phobia of hissing/air noises that is quickly linked to similar sounds is a lengthy and difficult process. Figuring out that your dog has linked the aversive to some other event or person and now fears them as a result, could take a long time to resolve. Your dog could now bite people without any warning.
But hey... its just compressed air, it's harmless...