I agree with your skepticism that celery derived bacon will have more nitrates. The nitrate amount in the final product is determined by the quantity of curing product used, and presuming that all celery based methods will result in more is impossible. You could surely game that by using more curing product than is strictly necessary on one side of the comparison.
But the reason you are not finding the hard numbers you want is that a simple celery based method doesn't have them, and never will. The amount of nitrate in a celery stalk varies based on how it was grown. You might be able to sample a given batch for its chemical properties, but you cannot predict them. To quote from an intro to meat curing: "There are absolutely no regulations or standards as to the amount of nitrate [celery] contains. Even if you use the same amount in every salami you make, you could quite easily be adding too much or too little nitrate to your cured meats."
Until Whole Foods publishes exactly what their manufacturing QA process is, we can't know how it compares to the well documented "pink salt" formula. I can offer a logical argument that there are probably more nitrates in the end result though. The downsides to using too much curing product are excessive nitrates in the result and too much "salt" flavor. The downside to using too little could be botulism, and inspection may reject it. If you're manufacturing food and the incoming strength of the nitrate is unpredictable, the case with celery, the obvious way to navigate that risk/reward tradeoff is to err toward using more than you strictly need. Then even if that celery batch absorbed a bit less nitrogen than average, you'll still be safe. But the average nitrate of that approach will be higher than a more predictable process.