His team had collected oral histories from several members of BoB. A lot of those short 5 or 10 second action clips came from someone's personal experience in battle. The Pacific was written a decade later, and a little tougher to collect those oral histories. It largely had to rely on the books written by two of the main characters (Leickie and Sledge). And you're right - there was no one unit that remained intact for the whole war. Survivors of Guadalcanal were being rotated home after being deployed for a couple years, and weren't around for Okinawa.