AFAIK, it doesn't say the toolkit is included, it says the "framework" is included:
>> ASP.NET AJAX is built-into ASP.NET 3.5.
http://www.asp.net/ajax/
So, you have the ASP.NET Ajax Framework available in 3.5 which gives you access to the client framework, UpdatePanel, ScriptManager and so on.
The Toolkit on the other hand is a separate download that gives you access to a whole bunch of other (AJAX enabled) controls...
Cheers.
Imar