I am claiming that statting up a *CORE* 3e NPC doesn't take a long time. Note the *CORE*. The assumption of *CORE* removes prestige classes and their prereq lists from the picture, slashes the list of base classes, decimates the feat list, shrinks or decimates (library dependent) the magic item and spell lists and removes a TON of other options. Statting up a core 3e NPC is easy.
4e doesn't have any splatbooks yet. Some are planned, and crunch work has probably been started, but for an early released adventure, you should stick close to the 4e core. So Mearls is claiming that 4e *CORE* NPC generation is fast. I believe him. *IF* he is comparing that to 3e *CORE* NPC generation, he is claiming that the later is slow. I don't believe that. If he is comparing 4e to late-years, large library 3e NPC generation, I don't care. Because I don't know how many options WotC will add in the splat-books. I do know that ze players love their options, and WotC holding a theoretical complexity line is, at best, a mediocre (non-zero until a substantial employee turnover) bet.