@Lougle Thai food in general does seem to call for sugar in many of its recipes - even if you're cooking from scratch.
The lesson is always to check the labels, on everything. I remember being shocked to discover that my jar of Chinese 5 Spice had sugar in it.
This is the ingredients list of the Schwartz one - note that sugar is the first ingredient!
Sugar,
Salt,
Onion Granules,
Dried Garlic Granules (12%),
Ground Star Anise (10%),
Ground Ginger (5%),
Ground Fennel Seed,
Ground Cinnamon (Cassia) (4%),
Black Pepper (3%),
Ground Cloves
... which is indicated by the fact that it's called Chinese 5 Spice Seasoning - but of course I doubt many people would realise that means it's not just spices.
(This is Sainsbury's own version list of ingredients:
Star Anise, Cinnamon, Fennel Seeds, Black Pepper, Clove)