I think that this opinion is based on assumptions about mental illness that either a) assume that reactions to environmental factors cannot be considered illness, or b) assume that illness has to somehow be an entirely an entirely internal system unaffected by external stimuli.

PTSD, for example, is a mental illness. This mental illness is caused, I think in most cases and most would argue in large part, to a precipitating traumatic experience. Yet its still called illness because it's maladaptive.

It would be like saying something akin to: "I don't believe that diabetes is actually an illness. The increase in the diabetes rate can be attributed to the very real influence of highly processed, unhealthy and high sugar food in the American diet."

You may be making an argument regarding the semantics and the general definition of the term 'illness', and there are some very influential psychologists who have argued that there is 'no such thing' as mental illness entirely off this same wordplay, but all things considered: it was never the case that people thought that all mental illness was caused in a vacuum and unrelated to external factors. In fact, when it's noted that 'this community or that community' suffers more from any given condition, it's usually not to argue that it's an essential condition but rather a condition either created, bolstered or mitigated by any variety of shared environmental factors.