Except they couldn't. At least not back in the day. If you want to say, "hey, we now have better capabilities and can detect changelings NOW," fine. As I've said now, countless times.
But you can't say, "oh, changelings never worked that way," because it's wrong. And, back during DS9's run, you absolutely couldn't discover changelings that way. Because the show couldn't have existed the way it did if you could have.
Dear god, please tell me you're joking.
BASHIR
(confirming)
When we scanned "Ambassador Krajensky"
for tetryon particles, he registered
as human.