I'm far from being an expert, but your conclusion matches my understanding, all this based on the post refered above.
Apparently, under 7.5v, you shouldn't connect a powered USB cable as well as external power. Although I don't think it can damage the board, the guy seem to metnion voltage drop and fluctuation instead. Above 7.5v, it seem that the board will switch power to Vin only.
If you REALLY want to be on the safe side though, I guess the best thing is to get a cheap USB cable and cut the red wire...