It is interesting, I just went out and tried to research if this was true or not. All I could find was sites asserting the same thing as you, but no one that cited a source. Again I am not saying one thing or another, but I would love to see actual research — or at the very least numbers.
The sources also vary between saying Celery Juice and celery powder. Like you (I am trying to pick on you) they say that celery is high in nitrates, but don't actually compare the nitrates in celery to the nitrates in the regular way in curing. No mention is given to factors that might affect how the nitrates are imparted between the two ways.
Maybe it is all a scam, lord knows we have enough of those in how food is marketed; but I am not seeing the data to make an informed decision.