Considering Jalal-al Din managed to with a much smaller force conquer and hold the Punjab region, and eventually Ogodei did send troops which were successful in Lahore and only retreated because he died, I would disagree with that sentiment.During early periods of Mongol Empire, conquest of India will not be successful. Mongol troops can't withstand Indian climate and disease. They just will die from heat and tropical disease. Now generations after when Mongols integrate Khwaresm, they can use troops from Afghan and Northern Pakistan in order to conquer India.
When Mongols invaded Southern China, the most troops were from China proper.