This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Ofcourse you can, if you make a class that extends Thread and implements Serializable, you can serialize objects of that class. The article you refer to doesn't say it's impossible to serialize an object that extends Thread.
But it doesn't make much sense to serialize a thread-like object. Serialization makes sense for classes that are containers of data, not for "live" objects like threads, sockets etc.