Perhaps, given Whitney's very severe ME he is reminding us that his illness is not invisible. One look at the pictures of him now show he's a very sick man.
Maybe 'hidden' would be a more appropriate word for those who are severely ill, since those of us stuck at home all the time are rarely seen by others, and can therefore be ignored and forgotten and the majority of the population are unaware of our existence.
For those with less severe ME, invisible might be a good word because when ME is milder we don't necessarily look ill. The person is visible, the illness is not.
In terms of advocacy, I think both hidden and invisible can be useful terms.