Again that article is about The Adventures of Ook and Gluk: Kung-Fu Cavemen from the Future not Captain Underpants.
Ook and Gluk is a Captain Underpants book. Captain Underpants is a series and this is part of it and has the main characters from Captain Underpants. The series (not an individual title) is #2 on the list because there are challenges to the books because of words and images that offend some people. Sometimes those people are liberals - and no I'm not saying it's 50/50. However, when Dav Pilkey gets his book challenged for offending people he has to issue public apologies and donate all revenue to a charity. Not so for any other title on that list.