1/5 of the general population do not develop mental illnesses within 90 days of an arbitrary starting point, unless the mental illness is extremely short-lived.
Think of the difference this way:
If 1/100 people are currently experiencing a headache, then it's also reasonable to think that 1/100 people will experience a headache tomorrow.
But if 1/100 people have an amputation, it does not follow that 1/100 people will get an amputation tomorrow, or even in the next 90 days. If 1/100 people were always getting an amputation within 90 days of any arbitrary starting point, there would be many, many more amputees around.
Mental illness being something that may lasts months to years, this certainly seems to be closer to the amputation case than the headache case. That implies that getting Covid can, indeed, be said to increase your chance of developing mental illness.
Think of the difference this way:
If 1/100 people are currently experiencing a headache, then it's also reasonable to think that 1/100 people will experience a headache tomorrow.
But if 1/100 people have an amputation, it does not follow that 1/100 people will get an amputation tomorrow, or even in the next 90 days. If 1/100 people were always getting an amputation within 90 days of any arbitrary starting point, there would be many, many more amputees around.
Mental illness being something that may lasts months to years, this certainly seems to be closer to the amputation case than the headache case. That implies that getting Covid can, indeed, be said to increase your chance of developing mental illness.