Is it true that the ACT doesn’t test any vocabulary?

Pretty much. The ACT doesn’t have any “vocabulary dedicated” problems, so you don’t need to know words like “ameliorate,” exacerbate,” and “truculent” to do well. However, the reading section still requires that you define certain words in context. The great news is that these words can be defined without knowing their exact definitions – you just need to figure out how the words are used in a sentence and put in another word that would fill the same role. There’s zero vocab studying required for the ACT, though having a big vocabulary is always nice (and is generally helpful in life). However, for efficiency’s sake, I wouldn’t worry about it much! There are much better things to study if you want to take your score up.