MAYBE it's one of those studies which has found a conclusion, which might be more circumstantial. The link between loneliness and health.

For example, there have been these studies where people have jumped to the conclusion that drinking small amounts of fresh coffee has health benefits - because people who do so have less stress or similar.
HOWEVER it is also possible that people who drink this stuff, do so as it's part of their lifestyle - stopping off at an expensive coffee shop for that and a roll maybe - and they could have low stress anyway.
To put it in an extreme and rather silly way - say a large gym started giving away purple hats. Everyone who went to this gym then wore a purple hat - imagine then if some group of experts were to issue a claim that wearing a purple hat made you less likely to be in bad physical health, because look at all these people who wear purple hats, they're in good shape and some look like they've even been working out

So regarding the link to loneliness, well I don't know about anyone else, but I know that when I am on my own, I am less likely to look after myself, I skip meals, I skip sleep, I just cut corners all over the place. I can't be the only one!