I'm with Hugo on this one that it's a documentation bug on that page. It even states on the very first line of the BOL link provided (their emphasis) "Returns 1 if the expression is a valid date, time or datetime value"

Interestingly though, on the Date/Time functions overview page, it does state "Determines whether a datetime or smalldatetime input expression is a valid date or time value."

(http://msdn.microsoft.com/en-us/library/ms186724).