I'm not sure I would read too much into some of these findings. It looks like the study assumes a diagnosis of long COVID if someone had a cough or back pain or any of the 30 odd symptoms they associated with long COVID. Less than 10% of what the study counted as 'long COVID' actually had a diagnosis of long COVID by their doctor.
If you have a pre-existing condition, you're more likely to mention a cough or back pain etc. even though you're going to the doctor for a different reason.