Eh, I dunno, man. A board tape is completely removed from what an audience hears, so using them as demonstrations of how awesome a PA system was doesn't make any sense. You'd need audience recordings made with microphones to prove that point. Unless they want us to believe those are audience tapes? I don't know. Maybe they are. But they sound like board tapes to me, in the video here. There is no ambiance.
I would also have to argue that someone 300 or 500 feet from the source is never hearing the same thing someone at the source of the sound is hearing. Speakers are speakers, however you arrange or stack them. In an arena or a theater, there's reverberation bouncing off all of the many hard surfaces, and there's nothing you can do to avoid or "mix around" that.
Outdoors might seem better, but it's actually worse. The sound dissipates very quickly, low frequencies travel further than high frequencies, the wind can change the sound, and there's also reverberation outdoors, though it's usually not as horrible as something like arena reverberation.
I've mixed shows in huge, ancient stone amphitheaters, smaller modern outdoor amphitheaters, a tiny (concrete) amphitheater at Santa Monica College, in city parks, on city streets, and even a velodrome (a bike racing track, this particular one was built for the 1980 Olympics in Los Angeles), and I can tell you from painful first-hand experience that getting good, consistent sound outdoors is like trying catch the smell of daisies in a jar and save it for a Christmas morning sniff.
It's a cool hippie thing they built there, it's actually kind of awe-inspiring, that mountain of speakers. But they're still just speakers. I get the feeling the thing they built has been mythologized a bit. But I guess you'd have to have been in their audience in 1974 to know for sure. Though in 1974 I'd imagine that most of the Dead audiences were too full of drugs and/or herb and/or warm wine drunk out of one of those vile pigskin things they carried over their shoulders to really notice.