I'm not going to get very far in persuading rekrunner that distance runners could have benefited from ped's in the late '60's, so I'll give that a rest and change tack.
Rekrunner, you strike me as a guy who is pretty knowledgeable about elite running, so I might draw to your attention some other features that seem a little odd about Keino's 1500 at Mexico. You say that Keino's performance was not outside his ballpark best at sea level - true, though we are talking altitude here - but something else that seems anomalous is the performance of the rest of the field.
In a championship race, and a final especially, the margins of ability between the competitors are usually quite close. It explains why many races are now decided in fractions of a second amongst the place-getters. Keino's race was run off a very fast - near world record - pace. It didn't come down to a late kick. In such races, the whole field is usually dragged along; many of the runners will likely pb. A good example of that was Rudisha's wr race at London in 2012, where most of the field set pb's and almost all went under 1.43. Another example was Filbert Bayi's wr 1500 performance at Christchurch in '74, where the first four runners went under the previous world record. (Interestingly one of them was Jipcho, who set the pace for Keino at Mexico.) I can give plenty of other examples. Fast races by closely matched elites typically produce fast performances by everybody (except the injured or ill).
Yet at Mexico, none of the other runners in the race recorded a fast time or anywhere near their personal best. Jipcho - who ran below 3.33 at sea level in Christchurch - finished in 3.51 at Mexico. Everybody, bar Keino, ran far slower than their best. Another runner in that race was the Czech, Odlozil, the silver medallist four years earlier. He ran 9 seconds slower(!) at Mexico off a much faster pace than at Tokyo. His performance was typical of most of the field, who simply died at Mexico (some almost literally). Why is that? The only conclusion I can arrive at is that altitude greatly affected all the competitors - except Keino. But here's an interesting thing - Jipcho is Kenyan and altitude-trained. He finished in 3.51 - clearly affected by altitude. And this is a guy who ran later ran 1500 far faster than Keino ever did. He might not have arrived at that level at Mexico - but to be that bad!
So here's my point: how credible can it be that in a fast championship race everybody was way off their best and finished in slow times - except for the winner, who ran a career-best mark? Everybody, including another talented altitude-trained Kenyan athlete, was clearly affected by altitude - except for the winner. And he had been affected in his previous races. In every respect, I don't think I have ever seen a race like it.