How does a wireless and batteryless pushbutton work?
Schneider are being a bit coy about exactly what's going on inside the transmitter. You have linked to the CE certificate which isn't much use. The Wireless and batteryless
pushbuttons
Harmony XB5R (2017) is more relevant.



Figure 1. The system appears to be very flexible with one button able to actuate multiple receivers or vice versa. This opens up all sorts of possibilities for use on forklifts, trucks, etc., for interaction with fixed installations.


  The Harmony wireless and batteryless pushbuttons range enables remote control of
  a relay (receiver) by means of a pushbutton (transmitter). The control is by radio
  transmission where the transmitter is fitted with a electric generator that converts
  mechanical energy, produced when the pushbutton is pressed, to electrical energy. A
  radio-encoded message with a unique ID code is sent as a single pulse to one or
  more receivers located several dozen metres away (see figure A). A single receiver
  can also be actuated by up to 32 different transmitters (see figure B).
  In order to avoid any conflict of multiple transmission from different transmitters, a
  minimum time frame of 10 ms is required between each radio transmission.
  
  Depending on the application, a relay-antenna can be used to get round an obstacle
  that impedes transmission or to increase the range (see figure A and figure B).
  The possible distance (1) between a transmitter and a receiver is approximately:
  
  
  100 m/328 ft where there are no obstacles,
  25 m/82 ft if the receiver is installed in a metal housing or in a closed metal
  enclosure,
  300 m/984 ft if a relay-antenna is located between the transmitter and the receiver
  (receiver installed in a metal housing or in a closed metal enclosure).
  


If, as we seem to all suppose, they are piezo generators then an introductory article by American Piezo may throw some light on the subject:


  Multilayer Piezoelectric Generators
  
  Multilayer piezo generators consist of a stack of very thin (sub-millimeter-thick) piezoelectric ceramics alternated with electrodes. The electrical energy produced by a multilayer piezo generator is of a much lower voltage than is generated by a single-layer piezo generator. On the other hand, the current produced by a multilayer generator is significantly higher than the current generated by a single-layer piezoelectric generator.
  
  Because they do not create electromagnetic interference, multilayer piezo generators are excellent solid-state batteries for electronic circuits.
  
  Due to advancements in micro-electronic systems many consumer devices have decreased in size. Smaller electronic systems require less power to operate. As a result, solid-state multilayer piezoelectric generators have become a feasible power source for some applications.
  
  Current applications for multilayer piezo generators are energy sources for munitions and wireless sensors, such as sensors that monitor tire pressure in automobiles.


They also have their APC piezo calculator which you may enjoy. I haven't made sense of it yet.