> "We performed a prospective cohort study of a population-based sample of 2315 middle-aged (age range, 42-60 years) […] During a median follow-up of 20.7 years […] A total of 601, 1513, and 201 participants reported having a sauna bathing session 1 time per week, 2 to 3 times per week, and 4 to 7 times per week, respectively."

They are comparing the health differences of the number of sessions, using only people who claim to do sauna at least once a week over 20 years. They have nothing to say about lower frequency or no sauna at all.