I think “covered well” is being used here as a synonym for “covered at all”.

For example, the “social workers & care -> counsel parents with child rearing problems” category is considered “well covered” because there are 13 prompts.

It’s self-evident that the spectrum of work carried out by a social worker undertaking that work cannot be well represented by thirteen prompts.

I suspect if you really strictly defined “covered well”, to the extent that any normal person would use that phrase, the headline would be above 99.9%.