You are misreading the ruling. It doesn't say that Itu Aba (Taiping Island) isn't an island. What it says is that Itu Aba is a maritime feature that "cannot sustain human habitation or an economic life of their own". Every high-tide elevation of this sort is referred to as "rock" in UNCLOS.
Being a "rock" here doesn't mean it's not an island; it simply means the maritime feature concerned -- should it be an island, an islet, a reef or whatever -- is not entitled to the full rights (i.e. having an exclusive economic zone and a continental shelf) of a full-fledged island. The concept of "rock" here is not a geological one but a legal one.
Being a "rock" here doesn't mean it's not an island; it simply means the maritime feature concerned -- should it be an island, an islet, a reef or whatever -- is not entitled to the full rights (i.e. having an exclusive economic zone and a continental shelf) of a full-fledged island. The concept of "rock" here is not a geological one but a legal one.