This is a different meaning of "mystery" from that used in your post. The existence and activity of angels is a "mystery" because we know of them through divine revelation, from Scripture, rather than from observation or deduction. It's a "relative mystery" because we can, with our natural reason, comprehend the existence and work of "created spirits, without bodies." As you observe, the activity of angels in the Bible is within our understanding.
An absolute mystery would be something like the Trinity. We can state that the Father, the Son, and the Holy Spirit are three Persons in one God, but we cannot truly understand how this can be using natural reason. We can only experience it through divine grace, in prayer and the Sacraments.
Very good definition and explanation. Thanks.