'Sonder' is most certainly a word! https://en.oxforddictionaries.com/definition/sonder
Yes, sonder is a word. The definition used in the article is debatable as to whether it's a word. It's a word invented in 2012 by a guy that wrote a book about inventing new words for emotions.
>Coined in 2012 by John Koenig, whose project, The Dictionary of Obscure Sorrows, aims to come up with new words for emotions that currently lack words.[1][2] Related to German sonder- (“special”) and French sonder (“to probe”).[3]
While "sonder" itself is a word, it means something different when used on its own (without) [1].
0: https://www.duden.de/rechtschreibung/Sonder_ 1: https://www.duden.de/rechtschreibung/sonder