I kind of have to disagree with this statement, because I have 12" drivers loaded where I hit a -3db point at 1.2K and they stay that way up to about 2Khz before they start rolling off more, approx. -6db@2.8Khz (measured on my horns) it depends the on how the horn loads the driver.
If designed right, with the right driver you can get a 12" within -3db at 2K, and with smaller drivers it's even easier to achieve.

Bill W.