The quick answer is that we have to disaggregate the concept of choice and autonomy for a person with a serious, long-term mental illness because of all of the psychosocial factors that infuse diagnosis and experience. If you think about persons with disabilities in general, their choices are driven by poverty, isolation, stigma, loneliness, feeling that they are a burden and so on, as well as potentially being coerced. There's also the suggestion, implicit or otherwise, which the UN is worried about, that they're better off dead than disabled.
When you ask about autonomy, you shouldn't be thinking about it in the same way you would if a person is unencumbered by all of those barriers to participation in society. I don't have a mental illness today, but if you stripped away all of the underpinnings that I enjoy that are protective, then I don't think I'd have the same level of autonomy. I don't think I could truly make the same kind of choice with respect to dying that others who have not been deprived of those fundamental rights could.
The commissioner on human rights in Canada said, “Medical assistance in dying cannot be a default to Canada's failure to fulfill its human rights obligations” because “systemic inequality results in inadequate access to services” and "In many instances people, with disabilities see ending their life as the only option”.
The commissioner on Canadian human rights said that.