It seems (on the basis of what you say here; I haven’t looked at the actual study) as if everything is consistent with the following situation:
“Long COVID” symptoms other than anosmia/parosmia are caused by believing you have had COVID-19.
Actually having COVID-19 makes you more likely to believe you have had COVID-19.
This is how it comes about that “having COVID on average gives you ~0.2 persistent symptoms vs not having COVID”.
Does the study give detailed enough numbers to distinguish this scenario from one where the disease causes the symptoms by “non-psychological” mechanisms?
Thats a fair point. I don’t think the data does distinguish between the two so maybe I’ve overstated the case here.
I think it’s important to distinguish between “is consistent with” and “implies that”. I think the belief hypothesis should be given a much lower prior than just Covid causing long Covid symptoms plus some additional cases for belief on top of that.