Yet, I find a few sections in the handbook links and comment sections that show that "itu" can be used to function as "is", such as:
In this page for instance, "Unity is good" has neither "the" or "that".
To make things worse, the above article never expressly states that "itu" can be a verb, but it says that it can function the same as "adalah" in ex. 98. It's just not clear to me when to utilize this very confusing other usage of this word!
Most of the time it means that. In this example, Persatuan itu baik it is just a determiner for persatuan. In Indonesian, in the formal register, nouns need articles/determiners and that's the function of itu here. It just refers to the concept of unity. You can just make this guess because you don't really ever say that unity and you also don't have a verb.
In the example Harimau itu binatang liar, it can be replaced with adalah, but that doesn't mean itu = adalah, it just means that the sentence transmits the same idea. Most natives would simply not use any of them while speaking, but you'd use either of them while writing an essay for example.