While the sample size is too small to be significant for a study, the results don’t surprise me because of the effects of cortisol on the immune system.
Also, if coffee impacts one’s sleep, that will affect the immune system.
Personally, I find that most of the time, adding fat to coffee (the heaviest cream I can get) mitigates its adverse effects. Every so often I cut out coffee for a few weeks or more if I feel that it’s negatively affecting me and then add it back in small amounts to see how I feel. I did so recently because my anxiety was high and I felt so much better. Now I’m drinking it again, one cup a day, and being mindful of how it makes me feel.
Bottom line: Drink coffee, but not too much, and only if it makes you feel good. Everyone knows their body best. As someone else said, the dose makes the poison. For some people, that might only be a little bit.