And there is nothing wrong with the idea that the Breen remain hidden that way, and that an occasional witness can utilize more elaborate means. The fault lies in Worf's babbling about the witnesses when the only thing he should be concerned with are the means.
That is, if it's possible to get through the Breen defenses, then the techniques of that, and the opportunities for using those, should be discussed. Witnesses would be a secondary issue. "It's too bad that Colonel Kira fumbled her chance to study the matter properly when she was a surviving witness to the Breen. She should really have used a tri-isopolarized frammistat there to see through the suits."
Even this would be intolerably illogical, because it would suppose that Kira indeed had failed to exploit these "elaborate means". Why should anything about the Breen remain mysterious after the "Indiscretion" encounter? Kira stuns a Breen, pops the helmet with the intent of using it as a disguise (as well as interrogating the prisoner), the Breen evaporates... And Kira leaves it at that? At the conclusion of the episode, she holds captive a shipful of these people. She is about to deliver them to the authorities - or to execute them on the spot since the authorities are not aware of the encounter, she's angry at them, and she's ruthless. Why not
keep popping a few more helmets, until the surviving Breen agree to telling how this can be done without killing them?
And if you think Kira (and Dukat!) too good-hearted for it, let's remember the Dominion held a Breen captive in "In Purgatory's Shadow". There's no good reason for Weoyun not knowing. And that can't be an isolated incident of a Breen being captured alive. The Breen and the Cardassians are sworn enemies from way back - why haven't the Cardassians tortured a Breen into revealing the secrets of the species?
None of that would be a problem if not for the nonsense we get in "Till Death Do Us Part". In the other episodes, the Breen are not
a mystery. Bashir knows that they don't have conventional blood. In addition to such fairly solid facts, rumors about them abound, but these also include detailed specifics such as the Breen having statistically more teen pregnancies than the humanoid average.
In the end, we're thus perhaps still best off if we assume that Ezri is being a silly little girl, perhaps due to starvation and Breen drugs, and Worf just plays it along, in the hopes of not sending the Trill into even worse fugues...