There must be something in the chestnut flower extract other than tannin. If tannin is all that is needed to protect wine we wouldn’t use sulfite.
Does that imply that both acids and tannins need to be high in the must? In the finished wine?
It's not an either-or situation -- it's a lot more complex than that. Wine longevity is affected by higher levels of acid, tannin, sugar, body. Plus K-meta. And all do different things and can work together. Wines with better longevity typically have higher levels of most or all of the items I listed.
Thinking of the cider David (
@Rice_Guy) mentioned, it has higher levels of acid, tannin, and body than say a typical apple wine, so it has better longevity.
There are numerous types of tannin, and they have differences. It may be that chestnut tannin has a higher level of "preservative" property than others. Or it may be that anyone using chestnut tannin (or any other tannin) simply noted better results when using it, and attributed it to that specific type.
Would I use a higher level of tannin and skip using K-meta? Nope! The notable problem with wine is that by the time we notice something isn't working, it's too late to fix the problem, e.g., once the wine is oxidized, well, it's oxidized. By continuing to add K-meta, I'm addressing one of the possible problem areas, and in my experience, it works so I don't change it.