This isn't really a bug, you'll notice that the CSS3 selectors also don't include a not operator. Instead, you should use :not:

Code:
Ext.onReady(function(){
    
    var form = Ext.create('Ext.form.Panel', {
        renderTo: document.body,
        items: [{
            fieldLabel: 'Foo',
            name: 'foo'
        }, {
            fieldLabel: 'Bar',
            name: 'bar'
        }, {
            fieldLabel: 'Baz',
            name: 'baz'
        }]
    });
    
    console.log(form.query(':not([name=foo])'));
    
});