This represents one of the classic Lojban fallacies, and even JCB believed it for a time.
There is nothing inherently "primitive" about most of the gismu, nor are they intended to be so. As an obvious example, nanmu means "male adult human", and could have been represented as "nakni je makcu je remna". But it isn't, because it is a sufficiently high-frequency concept that assigning a gismu for it made sense even in the Elder Days.
Actually, there's no implication of "adult" in nanmu. Keywords Considered Harmful.