I say this non-facetiously: any startup that looks like it might be hitting 1,000,000 jobs will be quite visible to YC as a good candidate to hit that number. They don't need to know how many jobs are created by most of their companies because it doesn't matter.
Also, once YC funds a company, they don't care what goal the company originally planned to hit during interviews.
Only a select few of the largest companies in the world employ 1,000,000+ people. The chance that a startup company will displace an international conglomerate like Walmart (most of whom's employees work for very low wages) is nil.
Walmart was a startup once. Starting a company to take on an entrenched giant is not rational. But I don't know what else I would do. It's right for me to take on Thomson Reuters and LexisNexis. Probably not for everyone.
Creating jobs doesn't necessarily mean employing people. A marketplace could create jobs by facilitating freelancer/client relationships which were otherwise impossible or unlikely to happen.
The criteria on the YC website specifically says they aren't talking about employing 1,000,000 people. They are talking about creating new things for people to do to earn a living.
Also, once YC funds a company, they don't care what goal the company originally planned to hit during interviews.