A TMSI is a randomly generated 32 bit number assigned to a mobile device, allowing it to be paged without using its permanent and immutable IMSI number. This number is assigned to a device by a Visitor Location Register (VLR) when the device enters (or is switched on within) the geographical area covered by that VLR. The assigned TMSI remains the same until the device connects to a base station under the control of a different VLR.
From the description of Footpath on the page you linked to, it sounds like these systems are used in shopping centres (malls), which are spread over a reasonably small area. Much smaller than the area an individual VLR will cover. This would mean that the TMSI would (with high probability) remain the same for the entirety of a shopper's visit to the area, allowing them to be anonymously tracked as they move around the centre.
While this technology would probably not be useful for tracking multiple visits of the same customer, analysis of the data could produce information on trends and patterns in customer actions.