That sounds pretty convoluted. Worf knows Breen evaporate at exposure => he quotes a rumor that Breen appearances are kept secret by all the witnesses dying before they can report?
What are you even talking about?
Worf: "They say no one has ever seen one and lived to speak of it."
Worf simply said that no one saw how a breen looks like. That is it.
As for the rest - the conspiracy theory has as much weight as an urban legend.
Maybe the breen kill all who saw them, maybe no one managed to see them in order for them to bother, maybe they just do not care either way. None of these possibilities contradicting my explanation in the least, Timo.