I couldn't find any mention of near-IR in the article, but supposing they are using a near-IR sensor:

Near-IR is a reflective imaging band. It does not measure heat unless the object is several hundred degrees C. It typically extends out to about 1000nm or so. To detect the IR light coming from a warm object (such as a human body, ), you need to detect light starting at about 3000 nm.

If heat cannot be measured (and remember we don't know that for sure since I can't find it in the article), they must rely on the skin being partially transmissive at longer wavelengths. Therefore, a dead severed hand will look the same as a live one (yay!).

Also, it would be much easier to just take a picture of someone's hand using a CCD with similar IR color filter, print it out (in black and white toner) on a piece of paper, and show it to the camera to get access. That's a lot easier and much more covert than chopping off a hand. You could take the picture with a tele-photo lens, IR filter, and security camera from several hundred feet away.