While this is partially true, it is not entirely correct, for it overlooks grammar. While it is certainly possible to take a newspaper written in Mandarin and read it out using Cantonese or Taiwanese pronunciation, the grammar is still Mandarin grammar, not Cantonese or Taiwanese.
Regional varieties of Chinese also have their own idiosyncratic lexicon, and sometimes it is not easy to represent these words using the standard characters. I have mentioned innovative characters for the Taiwanese equivalent of 的 (here) and also the use of 'A' in Taiwan to represent another Taiwanese morpheme (here).
In Macao, I saw this sign, where the Chinese is literally "not allow park car".
The third character 泊 is a local innovation, as the standard Chinese for 'park car' is 停車 rather than 泊車. The latter is a borrowing from English, as 泊 is pronounced [pak] in Cantonese. But it neatly illustrates that the use of characters in Hong Kong and Macao may not be exactly the same as in Beijing.
