"HP does not agree with [the study's] conclusion or some of the bold claims the authors have made."
HP doesn't really claim that its printers don't release ultrafine particles (though it's important to note that the study showed that many HP printers didn't release any emissions at all), saying that they are "consistently below recognized occupational exposure limits."
Rather, HP is taking issue with the notion that these emissions are dangerous. Here's what HP says: "There are no indications that ultrafine particle (UFP) emissions from laser printing systems are associated with special health risks. Currently, the nature and chemical composition of such particles—whether from a laser printer or from a toaster—cannot be accurately characterized by analytical technology."