This has been said because Ext is a very high level language for Web development. It is almost DSL like versus offering low level functions that you can sprinkle in your code. If you buy into the Ext way then you can build UIs in a very simple manner through JSON. This is all perfectly setup for a design tool, and with Ext Designer, they have it.
jQuery library simplifies the process of traversal of HTML DOM tree. You can use jQuery to handle events, perform animation, and add the ajax support into your web applications with ease.