You are using staging server - a separate instance of the ESP Component Registry that allows you to try distribution tools and processes without affecting the real registry.
# Component idfx ### C++ component library for esp-idf projects. The purpose of this library is to make it easier to use C++ to create an esp-idf project for the ESP32 microprocessor. The prime motivation was to make it easier to manage the Modulencer project, but also to share what I learned about using esp-idf.
idf.py add-dependency "skibu/idfx^0.0.1"