You are right that the demo must be updated, so that the UseSubmitBehavior property is available only for the StandardButton type.
Regarding your question about the skinned button rendering, this button type is based on the rendering of ButtonType="LinkButton" that also doesn't render an input. What has the skinned button compared to the link button is additional CssClasses and CSS3 (e.g., border radius, gradient, etc.). If we change, however, the rendering of the skinned button, for example to button HTML element this would be a breaking change that is not currently required.
If you want the input type=submit to look like the skinned button, you can use the RadButton's styles. For example:
"RadButton RadButton_Default rbLinkButton rbRounded rbCustomSubmit"
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