Just because they say ‘no visual cue’ does not mean that is no visual cue.

It literally, explicitly does, because they are talking about a hypothetical situation where no visual cues are left. If no visual cues are left, then there are no visual cues to see.

Why, because you say so? Yes, I can. Of course I can.

Okay. I should have been extremely specific. You cannot rightly and correctly say that there are visual cues that could be found when the other person explicitly says that there are no visual cues to be found, because in the hypothetical situation that they’ve brought up, there would be no visual cues to find, and so while you are physically capable of stating the phrase “just look for the visual cues,” or some variation thereof, you are incorrect in the assumption that there would be visual cues to find.

When somebody says “you can’t say” followed by a statement that’s incorrect, they aren’t trying to tell you that you are physically incapable of saying that statement; rather, there is an implicit “correctly” or “honestly” between the “can’t” and “say.”