Hello jefle,

It was added as a relation, seen as a practice in some countries, that even relatively small islands (without further elements, e.g. landuse multipolygons inside) were rather treated as island multipolygons.

But after multipolygonizing several such islands, this practice was rather discouraged by other mappers. 

I think the guideline shall be the multipolygon and place=island documentation of OSM wiki. It is not 100% black & white in this question. But whatever applies better to this island, it can be applied so.