That article did not completely decouple the effect of pH from the buffering effect of various counter ions. It correctly observes that mineral water does not cause much tooth decay (hydroxyapatite dissolution rate).
Another study from 1999 compared orange juice at pH 4 with orange juice added with calcium and found the same results. pH is one accelerator of erosion but it can be balanced by buffering effect from mineral ions in the drink.
The main conclusion is not really a surprise (especially if you think of the dissolution reaction equilibrium). A mineral water rich in calcium is basically not going to dissolve hydroxyapatite (a calcium phosphate) much due to the buffering effect of calcium. So it’s true that it’s a much better alternative as far as limiting tooth erosion goes. Same goes with orange juice with added calcium.
But I’m guessing as far as sodastream use goes, people are not starting from commercial mineral water in general (otherwise it may not be that much of a money saver lol). If you do, then I think you’re probably fine. But unless you have high calcium content in the water you’re going to carbonate, the carbonation is still gonna increase tooth erosion.