At low frequencies, the cabinet is relatively non-directional, as you say. At higher frequencies, when the drivers start beaming, the polar response of each driver is relatively independent of the others, so the total polar response of the cabinet is broader than it would be if the six drivers were mounted on a flat baffle. My personal experience bears this out. The rhythm guitarist in my second band had a concert Lead with a 610S.
I'm a little confused, though, when you refer to "the two up-and-down rows of speaker cones firing into each other". The cabinets are designed to be oriented vertically, such that the drivers fire across each other horizontally not vertically. Is that what you mean by "up-and-down rows of speaker cones"?