That's weird, it shouldn't!
Try clearing your cache to ensure that your browser hasn't cached a previous redirect.
Here's an example demonstrating that it does work.
I created a directory entitled "directoryIndex-example"
In this directory I placed three files, "index.php", "donhua.php" and a .htaccess file.
The .htaccess file contained this:
Now, if you go to the following url, you are directed to donhua.php
However, if you navigate to either file directly, then things still work as expected:
Saying all of that, I find John's solution much better, as it gives your visitors a choice in what they look at.
Also, remember that iOS devices don't support Flash.