How is carbonated water bad for your teeth?
Sodas (the sweetened ones), which I’m guessing are what GP is mostly referring to (what with the mention of sugar earlier in the paragraph) have sugar, which may encourage the growth of bacteria that are detrimental to teeth; and some of the flavorings are more acidic or react chemically with enamel to dissolve it.
There's usually a pile of acid added for "tartness" to sodas that lower pH, which overwhelms the buffers.
Often, it's phosphoric acid. Phosphoric acid will quickly react with any free calcium to form insoluble calcium phosphate which will mostly go right through you. So not only are you robbing your mouth of calcium you're also robbing the re-supply process.