Not to be a pedant, but wouldn't thread-safe automatically follow immutable?

If a structure is immutable, how would you be able to fuck it up, even if you abuse it via threading? I would argue the relation is so strong, that something which isn't thread-safe cannot be immutable. But there may ofcourse be something which I'm missing.

Different languages and platforms handling different key concepts differently etc.

Is there some properties of Ruby which makes this possible, or gives you a need for a double-guarantee, or is the declaration here simply redundant?