The claim made is that between 6 and 100 might develop cancer
if the machines are working as designed. However, we have no reason to believe the machines are calibrated correctly or working correctly. The TSA workers aren't wearing radiation detectors nor are the people going through the machines nor is there any independent outside party systematically or randomly checking the machines. So if one machine malfunctioned such that it was generating too much radiation (which according to the article we have reason to believe happens sometimes) it might conceivably give cancer to thousands or tens of thousands of people before anybody noticed.
Hey, remember when the security folk used to tell you that x-ray screening didn't damage film? That was an outright lie, but they said it anyway because it was convenient. So I get that the manufacturer claims the number who'd get cancer today is from 6-100; what I don't get is why you'd choose to believe that claim.