It is true for most of the restaurants serving different cuisines.
Home-prepared Thai food is healthier than the food at Thai restaurants.
Replace Thai with Chinese, Burmese, Vietnamese, Indian, and it would still hold true.
Having eaten a lot of home-prepared South Indian food, I'm not sure I really agree. Home cooked food has the ability to be healthier than restaurant prepared, but I don't know if there's enough data to really claim one way or the other - homecooked is not healthier just by virtue of being homecooked.
For south Indian food the problem is excess rice and starch.
I'd say most vegetarian Indian food has the problem of excess starch and carbs. One potential reason for the wide prevalence of diabetes in India.