Oh I missed the Soloman Islands. But anyways Palau and Nieu are members of some UN committees, and they're not disputed in any way, so it makes sense to consider them sovereign nations. Their situation is similiar to Micronesia, the Marshall Islands, and Palau, which are in free association with the US, although they are regular UN members.
"Free association" is like "commonwealth" a practically meaningless phrase. The people of Palau are citizens of Palau legally. The people of Niue are New Zealand citizens, just as the people of the Northern Marianas are US citizens, which is why I said Niue is more like the Northern Marianas than Palau politically.