@rrolo1 Flamme said "can't" not in the sense of individual capabilities but that, as a primitive, hierarchial culture centered on mana capacity, the demon society does not allow demons to downplay their mana without jeopardizing their survival.
Regarding where Aura had previously encountered an opponent who conceals their mana— it does not matter because this unknown individual clearly did not make a proper habit of it considering that fluctuations could be detected whereas Frieren and Flamme's technique does not allow that to happen. It's not unreasonable to assume that mana could be suppressed and there are instances in which doing so temporarily would benefit, such as reconnaissance, but what makes Frieren different from any other mage is the consistency of doing so purely for the sake of demon slaying.
On the other hand, normal humans aren't known for in concealing their mana possibly due to several reasons: 1) most humans don't have much mana to begin with, so concealing them doesn't make much of a difference against demons. 2) perhaps humans do conceal mana, but due to the same reason, the demons simply do not notice. You must realize that Frieren's advantage here is not just her deception but also her longevity. On the other hand, while we don't know much about elves or their practices, we can infer that they present a threat to demonkind,which is why they were pretty much wiped out millennia ago.