Actually everyone has some form of metaphysical beliefs, whether explicit or implicit. Even a hard-core atheist materialist who does not believe in any form of survival of consciousness is simply stating their metaphysical position, ie a form of nihilism, which may be true or may be delusional and not correspond with how actual reality works. So one cannot say that metaphsycial beliefs per se are 'delusional', since metaphysics simply concerns views about the nature of reality itself, and the relationship of ourselves to it.

There is definitely the possibility of delusional metaphysical belief formation using psychedelics, but there is also the possibility of increased insight into the nature of reality.

Psychological insights, which they define as updates to self representation, are generally easier to integrate so more likely to lead to beneficial change.

The study actually applies the falsity view to both constructs, not only metaphysical beliefs. From page 7:

"However, this does not imply that the psychological insights derived during psychedelic experiences are accurate. Previous research has shown that the experience of insight can occur in the context of false information (Grimmer et al. 2022). According to self-binding theory, weakened prior assumptions are the psychological mechanism that increases the potential of having insights during a psychedelic experience. However, this same mechanism is likely to also increase the potential of false insights (McGovern et al. 2023)."

Likewise, let us see what their actual definitions of positive and detrimental outcomes were:

"Beneficial and detrimental outcomes were comprised of enduring substance use change (e.g., alcohol, opioids, nicotine), lifestyle change (e.g., sleep, diet, exercise), and mental health change (e.g., increase or decrease in anxiety, depression, phobias, trauma beliefs, self-esteem, subjective well-being). These variables were coded as beneficial or detrimental depending on whether there was a reported decrease or increase in qualitatively positive or negative outcomes (e.g., a decrease in depression and an increase in self-esteem were both coded as beneficial outcomes)."

So one is more likely to reduce alcohol use, take up exercise and feel more general emotional wellbeing if one gains psychological insights, rather than if one comes to a specific metaphysical view for example that All of Being is fundamentally composed of One Infinite Mind. I somehow am not that surprised by the outcomes using these measures.

One area where I think certainly metaphysical belief change has shown to correspond to increases beneficial outcomes like reduced anxiety, is when psychedelics are administered at end of life care to cancer patients, and I think this is more an area where these beneficial effects would be more strongly apparent.