of course it's for the Pi, WiringPi is also for the Pi, as it uses (partially) the Wiring=Arduino
OTOH, e.g. looking at rotary encoder pigpio files
the examples are hardly understandable (not to say: confusing) for a hobby user, it's looking rather obfuscated.
Also code for more devices with convenient API functions would be wishful ( e.g., ads1115, mcp23011, pcf8574, CMPS12, mpu6050(dmp), mpu9250(dmp), bmp280, GPS Neo-6M, UART-communication from Raspi to Raspi or Arduino), providing tutorial-overviews and extensively commented source code also for teaching purposes
As to the library-APIs, the Wiring=Arduino philosophy for beginner-friendlyness also for Pi-users is the crucial idea.
http://people.interactionivrea.org/h.ba ... ow_res.pdf
The objective of the thesis was to make it easy for artists and designers to work with electronics, by abstracting away the often complicated details of electronics so they can focus on their own objectives.
As to the TOP, I still use WiringPi and IIRC it's finally still part of the Raspi OS distros (Stretch + Buster, apt-package).