That was his point. The article said that a region it outlines has an economy bigger than California. But adding up the GDP [1] of all the states that overlap that region gives an economy about the size of California's.

Philly is in a state that overlaps the region, but is not in the region. Hence, adding up the state GDPs overestimates the GDP of the region by at least the GDP of Philly.

[1] technically, I believe it is actually called the GSP (Gross State Product) in this case.