Timbre is not only about harmonic content. There's also the envelope of the sound - how it changes over time. Violin pizzicato (plucked) sounds completely diffe

Timbre is not only about harmonic content. There's also the envelope of the sound - how it changes over time. Violin pizzicato (plucked) sounds completely different than violin arco (bowed), yet it's the same string being excited into vibration. Pizzicato is percussive and decaying, arco is more smooth and sustaining. Same for piano - try to imagine a piano sound that doesn't decay and doesn't start with a bit of percussive thump, that would sound quite different!