Can someone explain this to me about the Tibetan Glaciers?

By the definition the uplift would be a lower elevation than the original height there for no gain in elevation. Isostatic rebound is an affect of the weight of continental ice masses that push down the crust by the shear weight causing the land surface to rise in elevation. This is still not any higher than the original height of the land mass before glaciation. The peaks of the Himalayas show no surface erosion of the peaks, only the plucking of ice masses further down slope. There are methods for determining the age of earlier glaciation by cosmic radiation dating. In short the effect of the Indian land mass colliding with Asia caused the elevation of the Himalayas.