I wouldn't be surprised if these people had been playing since the game first came out in their respective countries. One/two year(s) leveling a few crafts up and the rest of the time just selling. Give me four years and enough playtime and i'll have 300M gil too.
Even with a few months and a 100 craft (or two), I guess it wouldn't be tough if you literally did nothing but craft, and had about 9-10 Jeuno mules to sell your NQs and HQs.