Well, some information on these websites is simply outdated, some things is just wrong. There is nothing surprising about. People from outside can't know things better than these who are regular on Bitcointalk.
Coinmarketcap.com 1. It did not recognise Bitcointalk as the largest Bitcoin Forum.
What Is Bitcointalk?
Today, Bitcointalk is one of the largest online forums where people can ask questions and discuss topics related to Bitcoin, blockchain technology and cryptocurrency in general. The forum was created by the infamous inventor of Bitcoin itself — Satoshi Nakamoto, in November 2009.
I think that by saying "one of largest online forums" they talk not just about Bitcoin forums. It's about all forums from all categories - cars, food, IT and all other stuff. Because probably there is not that many forums online with over 3.4 million registered users.
Offcourse, I may be wrong. It may be just nitpicking.