It's out there. He's right. It's not that there's "little aerobic benefit," it's that the benefit drops off.
I agree with several posters who have said we need to see some evidence for this. Not because I am claiming you don't know what you're talking about necessarily, but, quite frankly -- I don't know what you're talking about.
First of all, you need to explain what you mean about "aerobic benefit." The word "aerobic" describes the way your muscles are working, not a type of fitness gain. When I see "aerobic benefit," I think "greater capacity to perform aerobically" which includes a huge array of physiological changes.
Now, if two relatively novice cyclists jump on a trainer and one bikes for 50 minutes and the other for two hours, I AGREE that the second sees hugely diminishing (even negative) returns for exercise more than 30-50 minutes in terms of oxygen uptake and muscle growth. In fact, the second biker's leg muscles will go very strongly catabolic after about 3/4 of an hour.
But what about, say, the ability to burn fat preferentially over glycogen stores? Even a novice biker going for 2 hours twice a week is going to be VASTLY more efficient at conserving glycogen then the biker doing 1 hour sessions, after just a few weeks. I think that cardiovascular efficiency (I mean the ability to maintain the training-level-specific power output over a given duration at a lower heart rate) probably sees a substantial improvement as well.
All of this explains to the OP why it's best off to mix duration as well as intensity. Long bike rides have substantial benefits, but maximum overall fitness is achieved with training in multiple intensity zones.