Yes, Clarke won a lot of minor medals at international championships, set a lot of world records, and had by the standards of the day a long international career. Getting up to win medals at 3 Commonwealth Games in a row is admirable consistency. No one doubts that, least of all me.
But ... the specific proposition being debated here is that Clarke would certainly have won if Mexico had been at sea level. On the one hand you have the world records and Clarke's dominating form in 1967/68. On the other, you have Clarke's consistent record of not winning gold in other championships.
If Clarke had won Tokyo, or if he'd won at least one gold at the Commonwealth Games then the idea that Clarke would have won a gold medal at 1968 Olympics, had they been held at sea level, would be a lot more defensible.
I know that's kind of a subtle argument, but I think that even on letsrun it should be understood.
Here, let's take a counter-example to show how this sort of evidence works. If the 2007 World Cross Country champs had been held in more temperate weather conditions would Bekele have won ... Almost certainly. Bekele has both world records on the track and a dominating record in world cross in temperate (even hot, Lausanne 2005 for example) conditions, but the heat in Mombasa was extreme like the altitude in Mexico was extreme (for championship level distance races). This suggests Bekele not only can run fast (like Clarke), but also can win (gold).