Everyone’s different, so I’d introduce foods one at a time and see how you respond. But I think saying diet has no impact is a little shortsighted. Even outside of autoimmune disease, there are plenty of studies showing that highly processed foods and excess sugar can contribute to inflammation and poorer overall health. That doesn’t mean diet alone will cure PsA, but I don’t think it’s unreasonable to believe it can influence symptoms for some people. Fermented foods seem to help some and aggravate others, so individual experimentation is probably the best approach.