A few more thoughts on that:
An equivalent ceramic capacitor might cost only a few cents, whereas that PCB cap probably cost more than a few cents in PCB space. It can still be a net win.
First, the cost of the actual component isn't the only cost added to that of the final product. Every additional different component adds several costs to the manufacturing process. It's another part you have to order/qualify/stock, it increases your company's risk to obsolescence, it requires another reel on the pick-n-place machine... A board made with 50 distinct parts will cost less to manufacture than one that takes 100 parts, even if the BOM cost is identical.
Second, it may well be that FR4 and copper make a better quality capacitor than a 2 cent ceramic. I suspect this is true, since plastics tend to make more linear dielectrics than ceramics, and glass-epoxy is a plastic if you squint.
Third, doing it on one layer of the board like that allows higher capacitance per unit area than the more common case, where you're looking at solid plates on different layers, since capacitance is a function of plate spacing. (So is voltage, but with the low voltages used in most digital design, you're limited more by your board house's trace/space limits than voltage tolerance.)
Finally, I found
an EE.SE question about this.