Professional degrees are degrees that are designed to prepare you for a specific profession. Those professions typically involve government regulation, with said regulation commonly requiring the degree in question. It's often used to indicate M.D. and J.D. degrees, although it also includes a slew of others, e.g., nursing, accounting, education.

I'm aware of what they are (and in fact have one myself), but my point was that they're distinct from Ph.D.'s (so the title is misleading), and they can span across a broad spectrum of fields.

Sorry about the misunderstanding. At a guess, I'd say they mostly just mean J.D. and M.D. degrees. Those are the ones most commonly lumped in with Ph.D.s, probably due to the amount of time you need to spend getting them.