I tried without success to find a copy of the official Senate Rules, but everything I found didn't mention a limitation on the filibuster regarding a nomination. It's as if it's an unwritten rule or something. How else could Clarence Thomas have been approved?
Regardless, I'm now quite convinced that you're right, and I'm also convinced that the Democrats are no longer constrained by "unwritten rules." In fact, written rules don't mean anything to them anymore, as shown by the NJ Senate race.
I think the RATs may no longer feel constrained by unwritten rules because they realize that they're going down, if not permanently, then probably for a long time. Thus, there's no reason for them to pay a price to keep the Republicans bound by such unwritten rules, because there probably will not be a RAT president for a long time.