It then goes on to provide several data type examples, including a lot of "date" constants which are actually all varchar constants.
Blimey. That really is quite special. Presumably they were trying to say something that was actually true, but I'm struggling to think of anything plausible...
What that page is trying to describe is how implicit conversions of string constants to date are handled. These are very common in real code. For instance, you can have an INSERT or UPDATE with a string constant for a datetime column, or a SET. Or you use a DATEDIFF or DATEADD function with a string constant to represent one of the dates.
All these cases need string constants, because unlike Access, T-SQL doesn't have date constants. (And unfortunately, there are millions of lines of code in the real world where this string constant to date conversion is based on unreliable formats. *sigh*)
EDIT: And the page mentioned by sknox is guilty of that as well. Two examples use December 5th, 1985. The third example suddenly switches to December 5th, 2085. And the last example then introduces May 12th, 2085. At least, that is how I would interpret the given examples. (I am quite sure the author of the page would disagree).
Check http://www.karaszi.com/sqlserver/info_datetime.asp - even though it is quite outdated (the newer date/time types are not covered, as far as I know), most of the information is still very valid.