Yeah, that is an open discussion on the discussion page for traffic_calming - many traffic islands do not really calm traffic, but guide traffic.
In my opinion it is too much to ask of OSM mappers to draw a distinction between the function of a traffic island (and it may be a fluent transition / both at the same time anyway). Also, whether or not a traffic island actually makes the road narrower is something that can be derived from other tags, namely width.
So anyway, what I want to say is: there is no traffic_guide=island, so I expect that everyone just uses the traffic_calming=island for traffic islands of any sort.