First of all, is that even correct or are the maps on this article misleading? Did they not even really conquer that area, just make it pay tribute?
(Also I realize that the Delhi sultanate survived after this, I meant fall as in great decline).
The maps are mostly correct, but the important part is that they represent the kingdoms at their greatest expanse. The Tughlaq kingdom for example was at its greatest under Mohammad bin Tughlaq, and also went into decline with his rule.
By the time Timur arrived, the Delhi Sultenate of Tughlaqs was limited to areas around Delhi. It probably should be noted that the Delhi Sultenate was never quite steady.