First off, welcome to UT!
You pose a good question. In the scenario you've described, either foot can become the pivot. In fact, I've seen this play a few times. Righty picks up the disc with both feet on the ground, defender things their left foot is the pivot and then ... BOOM ... thrower executes a play with their right foot as the pivot (which gives them extra reach). Perfectly legal. A bit awkward for a righty to throw with a right pivot, but possible. Very sneaky...
Doesn't really answer the root of the question, but at least gives some insight into your example.
- Seppo #22