IIRC India's GDP was approx. 25% of the world's in 1700s compared to Thai 1% around the same time. In case of India it came down to 3 to 4% in 1947. So India faced quite a drop.
I don't think comparing an already declined India to Thai is a fair comparison.
India's economy didn't get a boost because there was practically no industrial policy. British authorities would get their raw materials at gunpoint from the colonies, ship it to Britain for processing, and then ship the finished goods back to colonies (which were artificially shielded from the international markets).
Before getting absorbed by East India company, some native kingdoms/empites did start with modernizing their military which indicates there would be a strong effort towards industrialization if it weren't for the British regulations.
Unfortunately not many people understand or appreciate how colonialism destroys a country and they just see the end result and claim that the citizens of the country were incapable of developing by their own.
Not only economical, but also psychological (as in lose of self confidence) and cultural (assuming own culture is somehow inferior to the rulers) destruction happens. It is not much different from abused people losing self confidence and blaming themselves and additionallyhaving to suffer victim blaming from others - just at a nation scale.
I don't think comparing an already declined India to Thai is a fair comparison.