I went to argue against it and yet here I am, arguing that it's correct, detachment's don't get the General's Ld if they're not in range.
NEB, pg. 39: Detachment uses Ld of parent unit if within 3".
BRB, pg. 49: "...if a unit of troops is joined by a character (or any other model with a better Ld), the unit uses the highest Leadership value available."
BRB, pg. 82: "All friendly units within 12" of the General model always use the General's Leadership value instead of their own when making any Leadership-based test (unless their own Leadership is higher, of course).
Using a parent unit's Ld is not a test for the parent unit, so the General's Ld never comes into play. Characters in units, fortunately, still do.