To be clear, in the 2nd edition of the Starting Strength book (I don’t have the 3rd edition), he wrote:
"As a general rule, the time between sets should be sufficient to recover from the previous set, so that fatigue from the prior set dose not limit the one about to be done. The heavier the set, the longer the break should be. […]
The time between sets will vary with the conditioning level of the athlete, in a couple of different ways. Rank novices are not typically strong enough to fatigue themselves very much, and these people can go fairly quickly, just a minute or two, between sets, since they are not lifting much weight anyway. The first two or three sets can be done as fast as the bar can be loaded especially if two or more are training together. More advanced trainees need more time between the last warmups and the work sets, perhaps five minutes. If doing sets across, very strong lifters may need 10 minutes or more between work sets."
I’ve seen him write in other articles to basically drive that first point home - that rest should be long enough to not detract from strength. I’ve never seen him literally say “lifters should rest 6-8 minutes between sets.” The only place I saw him come close to that example was in his “First Three Questions” article, while addressing what to address to break a plateau, he wrote:
"Strength training is not conditioning, and if you do not recover from the fatigue induced by the previous set, then accumulating fatigue limits your ability to complete the sets and reps required by the program. In a novice program, fatigue is not a variable we wish to introduce, because force production is the adaptation we want, not conditioning. […]
So you do whatever you have to do to make this happen, and it’s quite obvious that if resting 7 minutes between sets alleviates the fatigue from the previous set, which is necessary to complete all three sets of 5, then you rest 7 minutes. Maybe 8. We’ll get hot, sweaty, and out of breath later – now, we’re getting big and strong."