I guess I wouldn’t object to some dialogue like that, but I also don’t see it as necessary. I agree that if Una weren’t as tough as Spock, or if she’d just never shared this with him, he would have spoken up. Since he didn’t say anything we can logically conclude that she is, in fact, known to be that tough. Unless there’s some other canon showing that to be untrue, I don’t really see the problem with simply accepting that conclusion.

I feel like this is only coming up because Una looks like a normal human. But we know she isn’t, so there’s no contradiction that I can see.