How would the iron from nails of the barrels have gotten into the bottles?
Maybe the nails go all the way through the barrel slats.
Possibly something like galvanic corrosion; copper and iron and seawater can react.
I saw a bad case of something like that with a copper domestic water system; a bladderless galvanized tank corroded through like crazy at the waterline once the system got a few leaks.
It leeches in through the cork. Read a story one time about a diver drinking salvaged wine. He dropped dead almost at once after drinking it. Others noticed an almond smell from the bottle. Turned out that cyanide leeched in through the cork. Whatever chemicals or elements that are near the bottle in the wreck can come in.
Someone doesn’t have a clue about cooperage; barrel making. There are NO nails used in making barrels. The edges of the staves were shaved to a precise angle, by hand and by the practiced eyeball of the cooper, so that they fit together perfectly and formed a circle, with the two ends of the barrel mitered into place as the staves were fitted, and all held together by the hoops.
The wine was poured into the barrel via the bunghole, which was then closed by driving a wooden plug into it. No idea where the iron came from, unless the wine was fermented in iron vessels or something. Or maybe the vineyard soil had high iron content. FWIW.
Making champagne usually doesn't involve barrels at all. The wine is aged in the bottle.
Methinks somebody who is unfamiliar with Methode Champagnois just assumed that it was aged in barrels before it was bottled.