As reported in the Philadelphia Inquirer:
How many skeletons might remain buried? Possibly thousands, according to archaeologists, but no one knows. Historical maps are unclear on the cemeteries’ boundaries, but numerous histories portray the grounds as used first by Quakers and then by the poor, whose numbers increased along with the size of the city.
They quote the Philadelphia Yearly Meeting general secretary, who had heard nothing about this. The article also cites a 1880s article in Friends Intelligencer, the predecessor to Friends Journal.
https://www.philly.com/arts/schuylkill-yards-quaker-cemeteries-philadelphia-history-brandywine-drexel-20190502.html