I think you are correct that good geometry doesn't REQUIRE Ackerman but the gentleman is correct in stating that it is much preferable to binding & over taxing a servo.
By the way, making the horns "square" is not as simple as it seems. I did a Pup elevator where I wanted the cables to go to the sides of the cockpit so I used an angled horn, with lines perpendicular to them and pulleys to straighten every thing out rearward. The system works well and there is no binding and no Ackerman either.
daRock, the arm lengths (front to back) don't have to be equal either, as long as the line of action of the cables is perpendicular to the arms. My Siemens Schuckert has unequal arm lengths but the plastic guides bring the cables in at the right relationship and the tension does not vary.