Since this a question of quantity, not authority, google the Sozomen quote, and note how many Adventist websites cite it. (I saw: Sabbath Truth, and its Amazing Facts siblings; LLT; Project Restore, Pathlights and its siblings; SDANet books, personal pages, etc.)
The reason the quote is so popularly cited by Adventists is that it is included in four major GC-sponsored reference works: *SDA Handbook of Seventh-day Theology p.521, Seventh-day Adventists Believe… p.259, and the SDA Student’s Sourcebook . Questions on Doctrine *p. 152 also includes Socrates Scholasticus’ parallel quote. In short, the quote receives a wide circulation in Adventist circles.
Hey, djconklin, SDA2RC, and myself seem to know about it. Clearly, at least a few Adventists and former Adventists on this forum are familiar with it.
Actually, mine is more a question of authority. As I said, all we need is the Bible to show the case for the Sabbath; all of the sources you mention would collaborate on that point.