They mean pet textiles like dog blankets etc, I've seen the same rule in shared laundry facilities all over Europe. The hair fucks up the machines pretty bad and gets on the next person's washing. Sometimes there's a dedicated smaller machine where it's allowed.
One of our local hotels has all the regular signs around the swimming pool: "must shower before enter", "children must be accompanied by adults", "no diving" etc, in multiple languages.
Then there is the extra sign "do not enter the swimming pool naked". They only felt the need to put it up in one language: German.
