Yeah, the article really needs to specify what its definition of "megacity" is. The YRD is a patchwork of many different cities, towns, and villages, some parts of which are substantially less urban that others. If you take the train from Shanghai to Hangzhou, you'll pass by some farmland on the way. I'm sure the Pearl River Delta is no different.
The distances between any 2 of Shanghai, Nanjing, and Hangzhou on a map looks far greater than Beijing to Tianjin, or Guangzhou to Shenzhen, so perhaps Beijing-Tianjin and Pearl River Delta would qualify as urban conurbations in way that the Yangtze River Delta doesn't.
A fair amount of people commute daily from Tianjin to Beijing, so I think it is a fair comparison.
The commute (train) takes about 1 hour (per direction, so 2 hours per day), and is good for people with offices/homes near the terminating station, but not so good for people that need an additional 1-2 subway rides to their office.
One hour commute times are pretty typical for largish modern Chinese cities.
