Looks like very fine frit to me too. You can see a slight twist to the underside of the cushion from where they pulled it together.
I think this is an example of someone over analyzing something they know very little about. Sand art in bottles and this paperweight are as alike as apples and oranges.
I couldn't tell if the age estimate is accurate or not. I can't tell if the pontil is firepolished or pressed with a maker's stamp. I suspect the weight is not as old as they think because of the shape.