They're just simple IR sensors - they can detect when IR is shining on them, but not from which direction. Put another way: each one is the equivalent of a single pixel in a camera sensor.
Position is determined by watching the timing very closely of when each sensor detects the IR sweep, and then doing the math given those timings and the known orientation of the sensors relative to each other.
Not to be purposefully pedantic, so please excuse me, but aren't the small "dimples" on the Vive hardware technically very simple, low-res infrared cameras that are designed to pick up the Lighthouse lasers?