Playing the devil's advocate, I wonder how certain they are that the earliest shells were used for anything at all.
Shells of freshwater shellfish found in my area are commonly found in the shallows mud with holes in them bored by a worm that makes a hole perfect for stringing, through which the worm enters and eats the innards of the shellfish. As a kid we'd look specifically for the ones with appropriately located "accidental holes" that would permit stringing a few together. (Admitting that my sister was sometimes noted to be somewhat "primitive" among other things, it was usually her idea that I should find the shells in the mud. The "stringing" may have been just to provide an excuse to get me filthy.)
We had the advantage of good "string" but it would have been more difficult to make something suitable from "found materials," as the holes (and the shells) were fairly small.
Obviously, one needs the full report and peer comments to evaluate the credibility of the assumptions.