“RMT” is an abbreviation of the term “Real Money Trader”, meaning a person who would trade Real Money for In-game Items, like currency (Gil).

The article you were referring to mentions the amount of people who were involved in RMT Advertising (the chat messages which invites you to buy an x amount of Gil for dollars), RMT/Illicit activities (for instance buying Gil through RMT means, Hacking or cheating), and the people who were participating in the said activity.

If you believe that someone is cheating or you wish to report an RMT, you can go to the Support desk menu of the game and file a “Report Cheater” report. You can find it under the following menu in-game:

[System Menu] -> [Support Desk] -> [Contact Us] -> [Report Cheating]