Light penetration just means light/umols that hit a specific area.
If you e.g. put a 2nd cob next to the first in close proximity, but both at half the power, penetration would increase due to the higher efficiency and more light output.
Or one could argue if u put 4 in corners, compared to just 2, you have more angles for light to be able to penetrate the canopy, also allowing you to put the light closer. (actually one of the big advantages of led over traditional lighting)
Cost, totally true for DIY. But as for that cxb200 there could be 4x Cobs inside for that steep price (600$), compared to other similar panels.
I think nowadays one best bin cxb3590 is about 55$ for end consumers...