Two-hour ‘dose’ of nature significantly boosts health – study

Experimental Design
From the paper:

Exposure was defined in terms of the self-reported minutes spent in natural environments for recreation in the last seven days; and outcomes were self-reported health and subjective well-being.

I haven't read many research papers since I graduated, and in my classes we never really dived into statistical analysis or critiquing experimental design, so I may be way off base, but this struck me as a bit odd.

That the “threshold” was also present for those with long-term illnesses/disability, suggests that the positive overall association in the data was not simply due to healthier people visiting nature more often.

This is what concerns me about the design. I have a long-term illness/disability, and if I was asked during a flare how much I'd been in nature the previous week, my answer would be 0 hours- because I'd been too sick to get out of the house. And obviously I'd say my health was poor and I felt down, because feeling too sick to get out of the house sucks. On the other hand, if I was asked during a really, really good week, I might make it to 120 minutes total if I added up my walks. I'd probably still say my overall health was poor, but since I was feeling good that week my answer about feelings of well-being would probably be higher.

They seem to be relying somewhat on this particular aspect:

Two further control variables were particularly important. First, the survey asked: ‘Do you have any long standing illness, health problem or disability that limits your daily activities or the kind of work you can do?’ (‘Restricted functioning’: Yes; No = ref). Including this variable, at least in part, controls for reverse causality. If similar associations between nature exposure and health and well-being are found for both those with and without restricted functioning, this would support the notion that the associations are not merely due to healthier, more mobile people visiting nature more often.

I feel like this doesn't properly account for people with variable conditions like mine, and also assumes that everyone who is disabled has the same type/level of disability. An otherwise healthy person who uses a wheelchair because of a spine injury might identify themselves as disabled in the survey but answer high on the wellbeing questions, and also be able to get out in nature more easily than a housebound person who says they have poor health and wellbeing. So seeing an association between time in nature and wellbeing in the disabled population could simply be due to the variability of disabilities and how they impact someone's ability to be in nature more often.

So basically I'm wondering if anyone has insight into what I'm missing- my assumption is that because I don't have any experience with real-life experimental design, I don't understand how statistical analysis accounts for this sort of thing. I know that there is other research into this area, so I'm not actually skeptical that nature has a positive impact on various health metrics; I just don't quite understand how this particular study works to support that hypothesis.

Edit: fixed "hours" to "minutes." Units matter :D

The actual paper: Spending at least 120 minutes a week in nature is associated with good health and wellbeing

I posted the article and not the paper, despite the article implying causation when the paper says "associated," because I'm interested in what folks think about the difference between the two - do you think the article overstates the study's findings?

I'm also interested in hearing people's thoughts about the experimental design, but I think I'll put that in another comment because it got quite long.