That does not sound completely accurate.
My understanding is that the snail shell is primarily composed of calcium carbonate but that there is a protective outer covering.
The outer covering of the snail shell is the periostracum, which is mostly protein.
Drop the snail shell in acetic acid and the dissolving calcium carbonate will form bubbles of CO2 as it dissolves.
And you are correct........
All FW snails and mollusc possess a protective layer, this prevents dissolution in the much more corrosive FW waters.
Now there are plenty of snails in natural systems with high CO2 waters.......there are tons of snails, I do mean tons, in pH of 4.7-5.1 in Florida, you can see them all over.
So why do the snails in our tanks get them holes?
The only good hypothesis I've got is damage to the layer, fish, transport, algae holdfast(see BBA after the alga dies, it'll leave a nice hole).
There are a number of things that will harm the layer.........but the pH and CO2 of tank itself is not part of it near as anything in a natural system could ever suggest....
If you accept that pH and high CO2 do cause an issue, why doesn't harm the snails where they are from etc?
That's going to be a huge hurdle to accept that hypothesis.
I'll say that CO2 ferts and low pH once the layer is busted is not helpful, but they should not be the primary reason.
Olive nerites I've personally collected from Santa Fe river, the pH there is 5.0.
Other locations had CO2 levels of 25 ppm, and the entire substrate was covered in a dozen species of snail, clams, FW mussles etc.
The hardness was very low and in some places, very high.