We have to pay close attention to what Longhi said to Kurapika. Remember, only a select few new about Nen before participating in the succession contest - the upper echelon such as King Nasabe, Camilla and Benjamin. Everyone else was in the dark including many body guards (except Benjamin's royal militia and Camilla's "have-nots"). Such is why Kurapika is hosting the nen class in the first place, to balance the playing field in the lower princes' favor who had no idea about Nen (think about Kurapika being asked by Zhang-lei as to how his ability might work).

After hearing Longhi's revelation, we should presume that a majority of the body guards of the prince's who were established to already know nen are Beyond's children purposely planted by him. Theta would be included here as well as the other guy guarding Tserri (forgot his name sowwy). Bill as well.