Picture a conical horn of some defined angle. Now double its length. Its mouth area has increased, but the cutoff frequency remains unchanged. The lower edge of its pattern control has gotten greater, but that's not what the statement said. It said "A larger diameter round conical horn with more mouth area has a lower cutoff frequency than a smaller horn." which is clearly untrue.
And, if the length of the two conical horns being compared is held constant, but the mouth area of one is greater, then the one with the greater mouth area will have the higher cutoff (and wider coverage angle), not the other way around.
As far as waveguides versus horns, you guys need to read my explanation on my website. You may not like it, but that is beside the point. Its how I see the issues and this thread is titled "Geddes on Waveguides".