In the '60s (and beyond) Kirby’s style was Marvel style. Although he worked very hard, cranking out hundreds upon hundreds of pages a year, he couldn’t do everything. In some instances, say after his brief run on the Hulk in Tales to Astonish, he did layouts rather than full pencils for a few issues so a title would maintain the Marvel “house style.” He basically plotted and drew everything, though not with his usual level of detail. Mike Esposito in the case of TTA would then finish the pencils and ink them.
In the case of Kamandi and with the majority of his art, he did full pencils and the D. Bruce Berry (previously Mike Royer) inked them. That’s the way comics were made until the digital age, although some artists inked their own work.
Clink’s standard notation of “primary” and “secondary” generally denotes the penciller and the inker.