I think I've just answered my own question, I reckon the guys who wrote this article probably Googled 'anti-rootkit' or something along those lines:
http://www.google.com/search?q=anti-rootkitAvast wasn't very prominent in the results.
The reason I bought it up was because Avast is a very popular anti-virus and is quite high up on Google's search results if you search 'virus software' or even more-so if you search 'anti-virus software'.

I was just surprised it was omitted from their list.