Piano technique encompasses learning music notation, how to keep a steady beat, and the ability to play both hands independently of each other with proper hand position, tempo, dynamics and articulations.
Being able to aurally discern chords, play by ear and sight read music add greatly to the enjoyment of any instrument.
The elements of artistry and expression, while the most difficult to teach, are what makes music an art form like any other. They're the elements that set a good musician apart from a great one. The ability to communicate meaning through the fingers and "paint with sound" takes music to the stratosphere.
Music theory is foundational to learning any instrument. Scales, arpeggios, chord structure and composition are an integral part of learning music.