Can you refer me to where you got your info or where you are basing your conclusions on?
Given how easy it is to confuse true NTD with other diseases, unfortunately a lot of the info out there is based on incorrect diagnoses. It takes a microscopic examination to definitively diagnose NTD, and few people are able to do that.
So we're left with diagnosing based on external symptoms. Or perhaps treating with antibiotics that will eliminate impostors and seeing what happens.
But there is one additional distinguishing factor that has never been reported by any reliable source, anywhere that I can find. What is the timeline and progression of real and verified NTD?
You can at least look up anecdotal reports here and elsewhere, and you'll notice they tend to fall into two groups:
1) Progression from onset of symptoms to death in a couple of days or less (sometimes even hours).
2) Progression in weeks.
With very little in the middle. And with such a clearly different progression rate, these deaths should not be all attributable to the same disease.
Here is an example report with progression in weeks:
I have had the opportunity to observe what I believe is true NTD on several occasions, and it was very much like what the above report described.
On the first occasion, although it fit the visual diagnosis perfectly, I was aware that visual diagnosis is not accurate. So I allowed the neon to remain in the tank. A few weeks later it disappeared, presumed dead but I never found the corpse. And a few weeks after that, several more neons simultaneously started displaying the symptoms. This is a classic hallmark of NTD, that it spreads primarily by consumption of the corpse by other fish.
At that point, I still left the neons in the tank, but in a breeder box. When they started dieing a few weeks later, I removed each before it had a chance to start decomposing. There was no further spread of the disease.
So I'm as sure as I can be without a microscope that this was real NTD, and that this is how it progresses. Any time there is rapid progression I know it's something else.
Other experiences back that up as well. For example, in my neon tetras thread, I didn't mention something. In one successful batch, one of the neons had intermediate-stage symptoms of real NTD which I didn't notice until I got it home. Based on prior experiences, I knew I had time before it died and the disease would be likely to spread, so I left it in to see what would happen. The antibiotics did not cure it (another hallmark), with symptoms slightly worse after seven days, at which point I culled it.
As for the symptoms you described, many of those are fairly generic, and could be caused by multiple diseases.
One actually rules against NTD. Real NTD causes depigmentation with underlying cysts, which appear white and bumpy. Columnaris causes depigmentation with loss of the outer layer of tissue, which appears gray - and which is what you described.
This picture appears to show both NTD to the right and columnaris on the left:
Another picture which shows real NTD:
But sometimes it's still not so easy to distinguish between the two, so use the progression rate. You lost the fish within a few days. If it were real NTD, the symptoms should have already been visible and advanced at time of purchase, and I assume you would not have accepted the fish if they were all in that poor a condition.