That wasn't saying you have to level quickly, as they've already made leveling a solo thing, instead of requiring a party to level past ~15 at any reasonable speed as it did before Abyssea. That was aimed at Jile saying that they mashed the easy button on leveling. They really had no choice with how topheavy the game was getting, and with the number of new players being pretty low. Level Sync in 2007/2008 sort of held them over a couple of years, by allowing people at very different levels to level together, but that just allowed people in the middle of the curve to finish the curve together, you still eventually run out of people wanting to level any more jobs. Abyssea made it so new players could join in much easier, and the December 2013 update made it so they didn't need other players. It's all logical progression as a game ages and the flow of new players dies.

If leveling was still as slow as in 2006 and required a party, then new players would not be able to reach max level with any decent success, and joining friends that play would be nigh impossible.