This isn't a study. Looks like they're recruiting people for a study at the moment. They have anecdotal self reports that 30% - 40% of people with long covid felt better after being vaccinated, but they also say 10% - 15% felt worse. Seeing how subjective this is, I would wait for the study to happen, and for the dependent variable to be something other than self-reports.

Even so, for people who have been infected and don't have long covid, it's entirely reasonable to skip the vaccine, as far as we currently know.