It certainly could be either side of the interconnect that's causing the problem. The important insight is that no ISP is uniformly bad at either end, which means that the problem is not at either end, but in the middle. So this study has found congestion in the core (at the interconnects), precisely where it is not supposed to be, and has found that congestion to be so bad that it drives performance to sub-broadband levels.
Cogent and L3 are ISPs as well, so I think saying "ISPs are failing to provide broadband speeds" is fair. However, to add context, L3 had a blog post a while back explaining that they are standing ready to add more capacity for cheap and that the congestion is, in at least one instance (in LA), Verizon's fault. http://blog.level3.com/open-internet/verizons-accidental-mea...
Cogent and L3 are ISPs as well, so I think saying "ISPs are failing to provide broadband speeds" is fair. However, to add context, L3 had a blog post a while back explaining that they are standing ready to add more capacity for cheap and that the congestion is, in at least one instance (in LA), Verizon's fault. http://blog.level3.com/open-internet/verizons-accidental-mea...