They have help. Most of these large residential towers got experts from Hong Kong where 60 stories residential complexes are everywhere. I'd not be surprised a huge percentage of the labors (i.e. supervisor, foreman, etc) are imported from Hong Kong and other large cities with decades of experience working with high rises.
Also most new residences in China are sold without interior. You get a big open space with water inlet, gas inlet, electricity inlet, and plumbing outlet, that's all. You have to wire your own everything and if you factor these into construction time, I'd be surprised if you can raise a 30 story buildings in 15 days.