This seems to me a silly argument. I have encountered angels and there was no "testing" about it, and angel is an angel and it was as evident as a bear being a bear and a kitten being a kitten. If an angel was a class of angel called a teraphim taking on human form, the angel would be doing the Lord's work and if part of that work was for you to know then the Lord could manage that, if not then you would not need to know.
The angels I encountered did not have human form and I wasn't supposed to see them work.
God lives in different dimension than we do, our senses aren't made to penetrate that dimension. We know that based on that God is eternal and we don't have but a dim perception of what it would be like to not live with time. Angels are beings who are so different from us that we weren't created with the kind of senses that can fully understand. Scripture tells us of times that God opened up the senses of a human so he could see into the dimension angels live in. An example was when Elisha saw the army of angels on the hill.