You test the tape before sending it to Iron Mountain, and you just have to trust that Iron Mountain isn't degaussing them. You retrieve one tape from them, as a test.

This reads like one ships the physical tape? I have no idea so honest question

It's an analogy. Storing a file in Glacier is like shipping physical tapes to Iron Mountain. You don't expect to get most of them back but you expect that you can get them back if you want to. You can test this by retrieving a random or non-random sample.