If you pick up other symptoms as a result of a post viral condition this may lead to anxiety and depression. Constant bugs, nausea , migraines, stomach upsets wreak havoc with one's mental state. I would have thought that was fairly standardly obvious.
Sorry if that sounds grumpy but there is a constant attempt to pick apart this kind of research, where other surveys with equally small return rates, sample sizes etc. are often accepted . Women are more affected than men and - like CFS and ME - there is often a lack of desire to investigate/believe / treat. Since this is a data thread and we love data, anyone who hasn't read Invisible Women really should.
Doctors will be looking out for clusters of symptoms.
The control group in other LC surveys confounded data because they can never actually remove the fact we are in a pandemic and yet they can't remove anxiety and depression as symptoms because that would not help to understand LC (there are plentiful other studies on anxiety as standalone symptoms of eg lockdown). They also admitted in the survey that many of their control group might actually have had Covid so this is a survey at least of people who definitely did.
On a personal note, I am now 7 weeks post infection and still feel awful.