When you need to have cron execute a PHP script, you have to ask yourself this question:
Does it need to run as CGI (by a web server) or as CLI (from the command line)?
If you want to run it as CGI, then you need to employ an HTTP client that will act as a web browser.
The example you posted was using the CLI method where the command executes PHP and PHP runs the PHP script.
I have not taken a look at the PHP source, but it seems the original code is designed to work either way. However I found a place offering a modified version that assumes it is running as CGI.
Having said that, you can probably get by with the following command:
wget http://example.com/wp-content/update-feeds.php >> /home/username/cron.log
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