I assume you didn't read the article, since that's their exact point...
"Since underlayers are generally the only element of which traces survive, such doctrines lead to all-underlayer reconstructions, with the overlayers that were obviously originally present excluded for lack of evidence."
"Since underlayers are generally the only element of which traces survive, such doctrines lead to all-underlayer reconstructions, with the overlayers that were obviously originally present excluded for lack of evidence."