The total area of the Pearl River Delta is about 40,000 km^2... The total area of the Greater Tokyo Area is about 4,000 km^2... Seems like a pretty arbitrary comparison if you ask me.
The definition of the urban land for PRD and Tokyo as used by the World Bank report is ~7000 square kilometers (PRD) and ~5500 square kilometers (Tokyo).
They do not count the entirety of either of the two. For the aforementioned areas (that they deemed urban land), they estimated them to have populations of around 42M (PRD) and around 31M (Tokyo).
This area excludes Hong Kong and many other parts.
Grauniad did a very poor job at getting the World Bank report's message across.
Speaking about your other attributes... I think the most telling thing about the Pearl River Delta is that 2 of those 11 cities were governed by the UK until 1997 - and you still have to pass through customs and immigration to cross into and out of them...
The customs thing is interesting. Citation?
Also, those two cities I was talking about are Hong Kong and Macau. Both were governed by the UK from the end of the First Opium War (1842) until 1997, when control was handed back to China. The terms of the handover included maintaining independent boarders (and forbidding complete freedom of movement and immigration). You can read about it and the other terms of the handover here (http://en.wikipedia.org/wiki/Transfer_of_sovereignty_over_Ho...).
It's substantially easier to visit Hong Kong than China (visa at the airport vs. visa in your home country before you travel), unless you have an APEC card of course