If I am playing them, the difference is very apparent to me. If I am listening to a recording, I think I can probably accurately distinguish between flats and fresher rounds the vast majority of the time. I think it is certainly true that there are occasions in which flats and dead rounds can be harder to distinguish on record, or possibly fresher rounds if the player is specifically trying to make them sound like flats (rolling off high end, using foam, using palm muting/thumb plucking, processing in post, etc.). Also, I think the latter supports the suggestion that rounds and flats inherently sound different rather than contradicts it; if you take rounds and try to make them sound like flats, you by necessity are admitting there is a difference that you are chasing. Conversely, I think flats almost never sound like fresh or average rounds in almost any imaginable situation.