You have set up a false dichotomy. Why do you ignore that word "partially" in the sentence you quoted?
When someone proposes a partial cause, then pointing out the existence of other causes does absolutely nothing to challenge the premise that there might be multiple contributing factors.
This is not a particularly controversial idea among health researchers. I've been casually following obesity research for decades now, and one of the big themes I've seen is one of scientists being frustrated at their inability to get journalists and popularizers to give a proper accounting of the level of nuance in the knowledge we have. Your use of the phrase "the causal direction" implies that your knowledge is informed by these same popular sources that perennially frustrate the research community, because a proper accounting of the science does not permit the use of a definite pronoun in such a statement.
Because the magnitude of the effects isn't the same. The evidence is very clear - obesity _causes_ several negative health conditions. To suggest that crash dieting somehow reduces the relevance of those conditions is to deliberately mislead. Some links:
I would love to see any research you're aware of that would show evidence that crash diets cause significant health issues (I don't doubt that there are some diets that cause weight loss while being bad for health in other ways) but to suggest that the side effects of those diets compare in magnitude to the side effects of obesity itself is extremely misleading. If you have any reputable sources that provide evidence to the contrary I would be happy to read them, but I think you're misleading people, deliberately or inadvertently.
When someone proposes a partial cause, then pointing out the existence of other causes does absolutely nothing to challenge the premise that there might be multiple contributing factors.
This is not a particularly controversial idea among health researchers. I've been casually following obesity research for decades now, and one of the big themes I've seen is one of scientists being frustrated at their inability to get journalists and popularizers to give a proper accounting of the level of nuance in the knowledge we have. Your use of the phrase "the causal direction" implies that your knowledge is informed by these same popular sources that perennially frustrate the research community, because a proper accounting of the science does not permit the use of a definite pronoun in such a statement.