All acts are instrumental, and therefore are tools. Even unvoluntary acts, like sneezing or twitching, are instrumental, the first to respiratory, the second to muscular functioning. If somebody runs away with the money from the bank holdup, it serves his purpose [to get money]. To catch and punish him serves not his purpose, but ours. For him getting caught is not "a good". For us caching him is. That's why it is much better to use a framework in which the word "good" does not even enter. It does not have to be im-moral, just a-moral.
Goodness = utility. Good so far, but keep going.
Is the act of believing this truly good, or only instrumental for you? It must be the latter since "all acts are instrumental."
Since goodness reduces to simple utility, this belief is of use to you, but it may not be of use to others. So this belief has no objective utility or "goodness."
Your belief statement rises only to the level of a statement like, "I itch," a feeling of yours that's of no consequence to others. Neither is the statement a claim about objective reality. It's solipsistic.