Deep explanation on TVS diode voltage suppression?
that would make the clamping of the TVS dependent on the impedance of
  the source


A TVS diode cannot clamp a perfect voltage source; it requires that the source (an indirect lightning strike/stroke for example) has a source impedance. For lightning surge testing to EN 61000-4-5, the basic source impedance is 2 ohms so, if the surge voltage is 2000 volts then there can be a peak surge current of close to 1,000 amps. Many TVS diodes are rated for this current and do the job of surge protection very well (and quickly compared to GDTs and usually with a significantly longer lifespan than MOVs): -



So, it's feasible that the BZW50-12 is capable of protecting equipment from an EN 61000-4-5 surge of 4,000 volts. That's a pretty good device in my opinion but, you need to understand the source impedance.


  Does a TVS diode really work by voltage division


I think using the term "voltage division" is one level away from just saying it clamps at a certain voltage and that voltage depends on the surge current.