I wholeheartedly agree with Rusty Neon's point. What characterises a professional in a field is that the person engages in the activity for remuneration (whether part-time or full-time).
I would not expect my plumber to necessarily know much about the history of plumbing, nor about the various materials used in other parts of the world, nor even about the problems associated with below zero (Celcius) conditions (as these do not apply here). I would, however, expect him or her to be able to do plumbing.
If I consulted an author on plumbing, I would not necessarily expect him or her to be able to fix a plumbing fault, but would expect that an understanding of the principles of plumbing are understood and explained.
In each case, I would need to trust that, along with any human being I encounter, they are basically honest and moral individuals (and would not care a damn about their production of a signed piece of paper that claimed that).
Likewise professionals in tarot: they either read tarot, write about tarot, run courses on tarot, or use tarot in yet other means in which remuneration or other formal exchange is normally undertaken.
The plumbing analogy is of course very poor, in that plumbing requires that one has particular and specific knowledge, whereas this need not be the case with tarot with regards to readings, but rather be open to a developing narrative.