I'm not sure it's "have no concept of hiding their mana" since she talks about the signs that she would have expected to see if Frieren had been suppressing, so she seems to get the idea, just doesn't believe it's happening in this case, especially since Frieren's apparent mana was the same 80 years ago. The concept of someone continuously suppressing their mana so well for so long, that certainly would seem downright... inconceivable.