1) The source doesn't combine Guangzhou, Shenzhen, and surrounding cities like citypopualtion.de does. Otherwise Guangzhou would be here.
2) We can be pretty confident about projections made for the next 30 years.
3) However, on this time scale, projections will probably be very inaccurate. 80 years out, population growth matters a lot. Blantyre currently has a population of around 1 million, and is projected to grow to 40 million.