You have assumed that drugs would not have assisted athletes at altitude. That is only an assumption. It is based on the premise either that altitude events will not be enhanced by ped's, or alternatively, that to be altitude-trained is more effective than being on ped's.
Firstly, we don't know that altitude-trained athletes were not also gaining advantage from also using ped's - and we do know that ped's were available in the 60's, as used by cyclists, for example, even though you maintain that they wouldn't have been used by distance runners - and it is also logical to conclude that if an altitude-trained athlete used ped's they would benefit from them. Ped's, by definition, add to performance - whatever the level of the athlete. At altitude, the base level fitness of an altitude-trained athlete would be superior to that of a sea level athlete, and that level could be increased by ped's.
It is therefore absurd to maintain that Keino, for example, couldn't have benefited from the ped's available at the time; it is also wilfully obdurate to continue to maintain against published research and anecdote from athletes of the era (and I have known some of them) that there weren't any drugs that could have benefited distance athletes.
Furthermore, doping doesn't have to be prevalent to know that it still occurs; and it is naive to imagine that runners would know of athletes in their sport, as well as other sports, who were using ped's and not consider it a possibility for themselves also - even if at that time only a small minority would choose to do so. As I have said previously, I recall an English middle distance runner who admitted to doping in 1966 - two years before Mexico City. I also saw that his level had noticeably improved, even if I don't know what he was taking. What he took Keino could have taken - if he chose to do so.
Finally, you say ped's couldn't have been available or couldn't be effective - both assertions wrong - because Eastern bloc runners didn't do outstandingly well at Mexico. The Eastern bloc didn't produce many distance runners from the mid to late sixties - it didn't appear to be a focus of their national teams at the time - and none that I can recall were outstanding at sea-level in that period. If their doping programmes were more focused on strength-related events then their distance athletes were not necessarily receiving the kind of "help" that would enable them to overcome the considerable natural advantage the altitude-trained athlete would have had at Mexico.
The core of your argument is that there were no known ped's at the time that would assist distance athletes - despite the known existence of synthetic testosterone and amphetamines, among other stimulants - or if they were, they were only used by athletes in other sports and not in track. Both arguments are fallacious. Anything stronger than cough medicine can be a ped. That you don't know of it doesn't mean it didn't occur. Denial doesn't change history.