Boiling (100C) will kill most . But 90C is not hot enough. Why not boil your water for 10 sec and then add to the tea? In general for making tea use freshly boiled, well oxygenated water (dont boil the crap our of it as it looses O2). Most prefer hard water too. Personally I dont use tap water for drinking in Thailand (have seen some very ill folk). BKK water can be quite salty too, especially in the wet season when sea water backflows under the city.
Probably perception of your (likely accent) potentially with a bit of racism thrown in. Sad to hear so much silly racism and silly woke in some of the responses to your post.