I don’t know of any publications that clearly state that there are no universal psychological truths across cultures, but I am yet to find any reputable evidence that there are universal psychological truths across cultures. Hence it’s the null hypothesis that hasn’t been disproven. If it can be disproven, I’d gladly change my assertion, but it’s impossible to prove a negative like this because it is the null hypothesis - it can only be disproven.
Nothing can really be properly proved in psychology anyway because of how soft the science is but also because of the changing nature of the influence of culture on psychology. Even within the same nation or peoples, culture also varies over time and so psychology is always playing catch-up. Social media related mental health issues are a great example - a psychologist who’s been plucked from 1970 and dropped into 2023 would have no idea how to counsel someone on that issue because it’s an entirely foreign concept to them.
Psychologists can absolutely learn and become experts in other cultures, but I think it’s beyond the scope of a single human being to become an expert in every single cultural context that exists. They often become experts in the cultures relevant to them - for example, trying to learn the differences between city/regional/rural issues, trying to understand the needs of LGBTQIA+ people, or learning to better understand CALD communities they’re based in/near.