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Basically what I saw on that page is a less powerfull version of the already implemeted site-wide commenting system XOOPS is using, but it doesn't look exactly the same.
If you really insist on having the look he uses I would reccomend:
1) Using a theme that displays the center block group on the bottom by having it included in the footer instead of the header.
2) Asking that guy for the source code of his comment script (though it wouldn't be that hard from scratch since it doesn't seem to be tied into any user management system. It just reads the parameters and dumps 'em in a database apparently.
3) Convert that code into a XOOPS block to be placed and displayed in the center area.
Though I would advise you against allowing anonymous users access to such functionality. But if you must make sure you have some way of moderating and sanitise the input well.