Where this function is defined?
Those functions have nothing to do with the Ticker library. Those functions are defined in the ESP8266 core.

The example you cite has this at the top of it:


  Hardware Timer0 is used by WiFi Functions. We can use only Timer1. Use of timer instead of Ticker gives advantage of precision timing and You can get timer interrupt in micro seconds.


Note the use of "instead of Ticker" there. That says that the code below has nothing to do with Ticker.