The 2000 gp listed at level 6 isn't a total for their entire career up to that point. It's the amount of treasure they are suggesting be handed out to the whole party between the time the party turns level 6 and the time they turn level 7.

That said, it's not at all far off from the amount that 4 brand new characters would get from table 10-10 Character Wealth if they were all being created at that level.

In my opinion, if the party as a whole is below that number, you should really be planning to hand out some extra rewards to try to catch them up. If they're between that number and double or so, you're probably fine. If they're more than, I don't know, like 5x that number, I would slow down on the loot for a while.