Where does all the sediment come from to raise the level of the Earth six miles in all directions? C’mon.
It isn’t laid down in all directions. It is in basins in select areas. It is not an even distribution any different that mountains.
The Appalachian Mountains used to be at least as tall as the Rockies. Erosion over immense time wore them down. Material like that is carried by rivers into lakes and oceans. The weight of the sediment will push down upon the basement rock.
You have to think in geological time frames; times periods where continents move around the globe.