“Pretty sure ... no, make that 100% sure ... that the people who do this point directly to the verse from Mark quoted above and say that they are following the Bible, the Bible alone, and nothing but the Bible, sola scriptura.”
Now, dear Campion, I think you know that every claimed tradition in differing denominations is later justified by pointing to some obscure Scripture, or stitched together from disparate pieces of Scripture to back fill a practice.
So, of course, in this case they have something to point to in order to support their tradition. That is not unusual at all.
The fact remains that they will tell you that the Bible alone is their guide.