I just measured the A4 one I got, for which the reMarkable would be a replacement, and it is 391g. So pretty close in my view.

Some math:

A US Letter sized pad made of 20lb paper is 1lb per 100 sheets[1] so an 88 page pad would weigh 400g, neglecting the cardboard backing. If it were made of 16lb paper then you would get a bit over 100 pages in 400g.

1: 20lbs means 500 sheets of the basis size weighs 20 lbs. The basis size for US Letter sized paper is 4 sheets (22x17), so 2000 sheets weighs 20lbs.