If you read the article you'll see they did a prospective trial where they assigned either wait-list or volunteering to two groups. This wasn't just looking at correlations between volunteering and health.
I did read the article. The second study also raises many questions.
What is meant by wait-listing? Does it mean their future is unclear and for this reason could their stress levels be higher?
Was maybe tutoring itself the activity that worked well not the kindness part in particular?
But anyway intuitively I believe volunteering and kindness do have health benefits, definitely reduces stress, feelings of guilt and just in general makes you feel better about yourself. So from this mental state there might be health benefits.
And while chemical levels differed it is still unclear what health effects there were. Being "linked" doesn't say much.
Few as an example:
1. Individuals who do high risk activities might be less likely to do volunteer work.
2. Individuals who simply have bad habits in life might be less likely to volunteer.
3. People who die earlier won't even make it far enough to be in position to donate.
4. Poor people are less likely to be able to donate and can get less care for medicine, have worse conditions generally.
I see thousands of such examples, I don't think it's possible to control for all of that, is it? You would have to
1) List all of those things that should be controlled. I imagine already this to be an amazing feat.
2) Somehow gather all this data from people.