Here's their website http://www.abiresearch.com/home.jsp.
The first article only discussed smartphones but I think the second one has it right. Both VZW and Sprint sell a boatload of broadband cards and I would attribute that heavy data usage to those cards.
Your attribution of heavy data usage to broadband cards is also, apparently, what the research report cited in your links claims. However, ATT claims they are not seeing the same usage patterns. I suppose that could be due to radically different customer usage patterns on different networks, or maybe ATT is lying to justify poor performance. Or maybe the report got it completely wrong.
Interestingly, the synopsis of another research report from the same group claims the following:
"In 2009, AT&T's mobile network experienced a huge surge in mobile data traffic. iPhone data usage was the primary contributor to the traffic surge that year causing AT&T to carry more data traffic than any other operator."