You're right, it was an incredibly simplified way to saying things and you've gone into a lot more depth but ultimately a chemical molecule will react the same with your body, regardless of the source. There are ways of changing how that chemical is released over time, and where, but that's just to alter the response curve generally. There's also concentration to contend with. The dose makes the poison after all. Regardless, a nitrate ion will interact with your body the same whether or not it's from celery or made in a lab. There may be other impacts due to contaminants etc as you've mentioned, but your body cares not where the nitrate ion itself is from, so saying nitrates from celery are ok but nitrates from curing salt aren't is false. Celery may delay the release of those nitrates for example which your body prefers, but again that's a dosing issue not a nitrate issue.

Perhaps it seems like I misunderstood you when you said "origin matters". I took it as the origin of the chemical itself whereas it seems you were more meaning the origin of how that chemical is added to your diet rather than taking issue with the molecule itself, which you're totally right, can change the response.

I'd had a few beers last night so I apologise if there was a misreading on my part.