The calories are not the shocking part; the sugar is. Mango is already sweet, then the shop adds syrup, then coconut milk and evaporated milk on top of that. The Consumer Council test found one 330g bowl with 40g of sugar and one 600g bowl with 66g, which is 32 percent over the WHO daily limit of 50g in a single dessert. For comparison, tofu pudding (豆腐花) in the same test ranged from 0 to 5.4g per 100g.
As a tong sui it is middle of the pack on calories. A 350ml bowl at 254 is about the same as a bowl of taro coconut sago (262) and well under walnut paste or black sesame paste, which the same tests put at 130 calories per 100g, so over 400 a bowl. Grass jelly with all the toppings tops the list at around 430. The fat is modest at about 8g, mostly from the coconut milk, so this is a sugar and starch dessert rather than a fatty one.
The dessert shop bowl is the trap. Hui Lau Shan and the chain shops serve it in a 500 to 600ml bowl, sometimes with extra mango, ice cream or a second scoop of sago, and that is 400 to 500 calories and over 60g of sugar. The 250 calorie figure is a normal restaurant tong sui portion after dinner. If you are ordering it as a standalone afternoon snack in a big cup, count it as a meal.