that's not true. There are only a few countries in south-east asia with drinkable tap water: brunei, hong kong, japan, singapore, and south korea. You should probably not have drunk that water in Bangkok.
Even within a country, whether a specific city has potable water or not can vary. Bangkok technically has potable water from its source, but the pipes it might travel through can be broken and dirty (the same is true in Beijing, actually). In this case, you can get lucky...but it is better not to chance it.
