Never heard of it to be honest. Are they validating it?
I suppose if a manufacturer were making a seasoning they may use this method. What I'd want to see from them though is this:
A positive control test of the product containing allergens
Positive control swabs in key places when "dirty" (which should show the presence of allergen)
Testing of the last salt through on the purge, probably in triplicate to show absence of the allergen
"Clean" swabs in the same location to show absence of the allergen
Because it's atypical as well, they'd need to make sure they validated the purge time, flow etc and they measure this when purging. I'd also recommend some of those quick verification tests (like pregnancy tests) you can use to make sure the purge is effective each time. They're not as good as ELISA etc but would show up a gross issue.
If they have pipework they're purging as well, in the validation they need to be breaking into it to check for visual signs of any "hang up" points. Gut feel is that this is pretty likely in dry delivery systems.
I have to admit it would make me nervous too. I suppose because it might not take much for the flow of salt to be disrupted by a blockage etc.
Depending on what it is though there are other dry methods, e.g. alcohol based cleaning agents or wipes (quick evaporation). How many allergens do they process? Sadly this would probably be a case for "may contain" labelling (which I hate because my son has food allergies and if he avoided everything with "may contain nuts" he'd not eat.)