Aladdin Khilji did manage to defeat them. And this was the zenith of the Delhi Sultanate, remember. by the time Babur came, it had cracked into little pieces and was generally open to invasion.
My understanding is that it was more a matter of a mix of display of power (the Delhi Sultanate was quite a power at that time), unusually sensible and effective diplomacy, and some gifts/tributes, more than impressive military success by the Delhi rulers.
However, the Mongols had easier fishes to catch and fry.