I agree, the nut flush is defined as the best possible flush. However, we cannot call a straight flush the nut flush because it is
not a flush, it is a straight flush! A different type of hand (just look at any hand ranking chart).
If we have an ace-high flush on a non-paired board (no straight flush possible) we call it the nuts. When the board is paired, we cannot call it the nuts, but we can still call it the nut flush. Similarly, if a straight flush is possible, we can't call it the nuts. But we still call it the nut flush.
Obviously this is only semantics and isn't that important, but hopefully you can see where most of us are coming from. I say most of us because I found
this thread and most everyone agrees with me.