That UL statement is technically correct. It is also incomplete enough to be misleading, even dishonest. There are no, and never have been, UL-listed "tamper resistant" cover plates, as defined by the NEC, and marked with the "TR."
However, there are a variety of devices, cover plates, and inserts that were previously used - long before the NEC addressed the topic - to protect little kids from themselves. These items are still made, still listed - but not listed under that particular category. If you visit any 'box' store and examing the products, you will see that they do not have that "TR" marking.
Prior to the recent code change, there was no UL standard that addressed 'tamper resistance' for electrical devices or their covers. Keep in mind that any standard is no more than its' own particular requirements, and cannot address features beyond the scope of the standard.
