Although the article says that it is a habit typical of Shanghai, anyone who has been to other South-East Asian countries knows that "pajamas" are worn in public elsewhere as well. When I was young(er) and living in Saigon, there was really no distinction between what you wear to bed and what you wear to go outside. Unless you work in an office or go to school, most people on the streets wear clothes that are considered as "pajamas" by Westerners. The peasants working in the fields would be described as "black pajamas clad" by American soldiers and Newsweek or Time Magazine.
The thing is, contrary to Western countries, where different activities call for different clothes, people in Third World counties cannot afford to have too many sets of clothes and tend to wear the same type of clothes all the time, indoors or outdoors. In Vietnam, there usually are three types of clothes: formal wear (ao dai for example), everyday wear (bà ba, "pajamas", etc.) and Western clothes.