It seems the video is a bit dated. When I tested this in version 2001.2.1305, I was able to highlight a disabled Silverlight element directly and craft an advanced verification on it. There is no IsDisabled property; you'll have to selected the IsEnabled property and set it to Equal to False.
See the attached screen shot for an example against a public site. After navigation, I clicked the DataForm left-hand menu item. Then I clicked the IsReadOnly checked box. This disabled the Business Address CheckBox. I was able to highlight it and select the second blue nub which corresponds to the CheckBox.
Does this public example work for you? Are you sure you're targeting the correct element in your application? Can you provide access to your app, or a public site that exhibits the same behavior, so I can reproduce your issue?
the Telerik team