I find that article to be in need of review by someone with good literacy. It presents single datapoints and talks about "this decline", despite not actually presenting past data to compare with the current numbers. The largest timespan it shows is the literacy from '92 to '03, and they explicitly quote the report saying that "average prose and document literacy did not differ significantly".

They also fail to mention that the 2003 report showed no decline except for Hispanics. Maybe, just maybe, immigration by people with non-English backgrounds was the cause, not textbooks? It'd be nice to at least see that discussed.