I tend to agree with you.

Science can tell us how to most effectively maximize happiness so long as we have an operational definition of happiness. But it cannot tell us whether happiness is in and of itself valuable.

In other words, science cannot answer the question: Why should we value happiness? Suppose a meta-ethical moral nihilist says "The only thing I value is my own happiness". How can a scientist prove them wrong? They can't. Because science is not in the business of answering questions about value.

However, the mistake is to think that philosophy can better answer those questions. Even highly trained philosophers cannot "prove" to the nihilist that they should value other people's happiness. Ultimately value-statements are subjective. If the nihilist only values their own happiness - there is no logical argument that exists which can definitely prove them wrong.

All we can do is to take advantage of the evolutionary fact that our species is social and we are programmed to value other people's happiness so long as they are a part of our social circle. That's the only thing that is keeping psychopathic nihilism from spreading in the population. But suppose the population was composed of 50% selfish psychopaths and 50% altruists. There is no "universal moral guidebook" that can determine who is right. It's all subjective.