So why do people feel that differences cannot be heard now unless the room is optimized. First we had "There cannot be any difference" then we get "I can hear subtle differences" and then "nobody can hear differences unless the room is treated".

I know a well treated room sounds better than a non-treated room, but if the only thing being changed in the system is the cable, and whatever room it is stays static, then there is no reason the same or similar differences cannot be heard in a non-treated room. If the room was treated, the differences may be more profound and easier to hear, but I disagree that one needs a fully treated room to notice a difference in some cables, maybe not all.