Only versions of Windows Vista or newer will return a value to the WindowEdition conditional comment.
From the Conditional Comment Wikipedia page
An integer corresponding to the edition of Windows used to view the Web page. Returns a Boolean value of true if the value matches the edition being used. For information about supported values and the editions they describe, see the pdwReturnedProductType parameter of the GetProductInfo function.
emphasis is mine. By edition they mean Home, Home Starter, Professional, Ultimate, etc.
The MSDN page for the GetProductInfo function supports this. The page also lists the input parameters for GetProductInfo which include
dwOSMajorVersion [in]
The major version number of the operating system. The minimum value is 6.
Version 6 corresponds to Vista, so I suspect Windows XP and versions of Windows older than XP do not have this GetProductInfo function, cannot call it and won't return anything to compare to the conditional comment in IE.