Besalel, that’s not how safek derabonon lekulah works. It’s efshar levrurei, able to be clarified; it’s not like you have 3 pieces of meat and don’t know which one is treif,.or if milk falls into chicken and you are unable to measure. That was the most dangerous line of this threat posred yet and one of the most shocking suggestions I’ve seen on here – not knowing it a known individual is Jewish would not allow us to treat it as a safek. Only if his status were unverifiable would we treat him as a safek,
Also, chazal added many chumros to yayin nesech far beyond ordinary derabonon’s.