If loss of earnings has been caused by Covid then a self employed person can hand on heart say they were adversely affected by it.
One example I believe is loss of customers. And loss of customers may cause loss of earnings.
And it may not.
If you've had to stop, or temporarily scale down, your work, you would be adversely affected and eligible to claim.
But earnings are not being taken into account.
HMRC have given examples of what they class as adversely affected and loss of customers or earnings isn't one of them.
To me, it boils down to whether you've had to have time off from work because of co-vid.
There are other examples but, again, losing income isn't mentioned.