That's one possible explanation, but it's considered a fairly weak one. The more likely reason is fire.
But if this is wrong and meat did allow for increased human intelligence, then all that prooves is that meat was an integral part of the diet of earlier hominids like homo erectus. Homo sapiens have had access to fire for their entire evolutionary history. One consequence of this is that we have relatively weak stomach acid and can't even eat raw meat safely because our stomachs won't kill the bacteria that give us food poisoning. But fire doesn't just kill the dangerous bacteria in meat, it also greatly increases the nutritional benefit of vegetables relative to the energy expended in digestion, which means it's entirely possible to support the caloric requirements of a modern human body using only vegetables.
The only real limitation to a vegan diet is vitamin B12. B12 deficiency is responsible for almost all of the health problems associated with veganism, but can be entirely mitigated in most cases with supplements. It's also worth noting that a significant proportion of the human population lacks the ability to metabolize sufficient B12 from food regardless of what they eat. We don't notice in most cases because, unless it actually gets bad, it's just one of the many stresses that the human body is able to tolerate.
Humans are "supposed" to eat sugar. It's the only taste that is innately appealing to humans from birth.