Explanation: More a joke than anything - while Romans were immensely disdainful of non-Romans (including Italians, and even their fellow Latin Italians), to varying degrees, intelligence was not the usual line of denigration. On the contrary - Romans often appreciated the cleverness of conquered (or conquerable) peoples, precisely because clever people are useful (and profitable).
The Romans, rather, believed themselves to be a 'middle' people. As they lived in the most moderate and even zone of the world (Italy), it was their view that the Romans had become a moderate and even people, without excesses of any particular virtue or vice as a society. And moderation was a big value amongst the Romans. Overbrave men threw their lives away - overclever men outmaneuvered themselves with complex schemes when confronted with honest, straightforward behavior. Moderation was virtuous and powerful.
They did not consider themselves to be the most brave and warlike - they considered those weird pale folk from the north to be the most brave and warlike. They did not consider themselves to be the smartest - they considered those odd southern folk down in Africa to be the smartest. But they considered themselves to be the most suited to rule and to conquer (reasonably, moderately, and strategically, of course!).
How very convenient for their imperial ambitions!