please pardon my late reply. I caught a cold last week. ...
The issue I'm facing is not dealt with by KeyboardNavigationSettings, as this setting still doesn't enable Telerik controls to be reachable by the TAB key on the keyboard (which is an WAI-ARIA requirement). You can reproduce this issue from the example you have provided: Try to reach the control by hitting the TAB key - you won't be able to.
Actually, after fiddling around a bit I learned that in addition I need to set the TabIndex property to "0"; so Telerik controls will be able to gain focus by hitting the TAB key repeatedly.
Since adding TabIndex="0" is a requirement for being WAI-ARIA compliant, I propose to have this property automatically set (with value "0") when the TabIndex property is not provided and EnableAriaSupport="true".
Or there should be a remark on the corresponding help pages for EnableAriaSupport stating that it's s formal requirement to additionally add a TabIndex property to a Telerik control for them to fully comply with the WAI-ARIA proposal.