Was Dave Scott a vegetarian his entire life, or simply converted to it when he was a fully-grown adult? Most likely the latter.
Protein consumption as a growing child and teenager is directly proportional to height - it's the sole reason the average white male has increased 6" over the past 200 years. Genetics haven't changed, just our diet.
Dave Scott was awesome for sure, but what matters here are the Olympic sports and each one requires certain characteristics. You won't see a weightlifter competing in canoe, 5000+ running/walking or shooting for example.
Anomalies aside, it would be interesting to observe correlations between a largely ( >51% ) non-vegetarian diet and athletic predisposition and performance.
Holding every other factor (race, geography, culture etc) constant, the exponentially greater protein availability (in meat consuming countries) cannot NOT have consequences for general muscular build among the populaces and as a consequence, some form of athletic predisposition.
A good question to start with:
Are there any predominantly vegetarian regions - besides large tracts of India - elsewhere in the world?
Quoting wikipedia:
"During the period in which he won all six of his Hawaiian triathlons Dave Scott followed a strict vegetarian (vegan) diet."