I think it's similar to what you can buy yourself from Travelguard.
They can add it, as it's probably more convenient for most folks to have Disney do it than buy it yourself.
I think if you get it yourself from Travelguard, you may get slightly better coverage. They price it according to your vacation price. For us, we didn't include our airfare price, as we are flying Southwest. If we need to cancel, we'll just get a credit and use it later. If you buy it within 14 days of your initial deposit, Travelguard will cover preexisting conditions. I like that, as DS has asthma, and I worry about that being one?
I am a control freak, so I buy it myself. I think with Disney, you would still be responsible for a cancellation fee, but am not sure.
I have medical insurance, so they covered when my kids got sick at Disney (twice - ugh). They even paid for the doctors that make house calls to the resort - wow I was surprised that they covered that! And we have an HMO - not the best but not the worst coverage. The
travel insurance would cover any co-pays that the medical insurance doesn't. So you'd have total coverage. That is nice.
I would call and ask Disney myself.