The author just refers to JWs as they are the only Christians he knew of who didn't celebrate Christmas - he mentions it as he was surprised that this non-JW couple, attending his Mennonite church, also did not 'celebrate' Christmas.
Aside from meeting Jehovah’s Witnesses, I had never encountered Christians who did not celebrate Christmas.
Actually they did, but in a much more non-commericalised, more religious way - that was the point of the article - it lead the author to re-evaluate his view of Christmas too.