I don't think it's an "absolute, per se.
I think it's a general suggestion for all speakers, not just for their product. If two pspeakers are placed close together, stereo imaging will be severely compromised, particularly at their example listening distance of 9 feet. Obviously, near field listening has different rules and I don't think the 9' distance applies to that.
If you have these, you already know they image pretty durn good. Why impede it? But, if ya gotta place 'em closer together then I don'tthink they would suffer any more than any other speaker.