All the pictures did look posed but that would be the nature of the camera and filtering.
My dad was an avid photographer and was a photographer is US Army SIgnal Corps during WW II. He had old magazines with ads for color cameras from the pre-Kodachrome days. They were large and bulky. There was a prism or mirror which split the image to three seperate sheets of film and the light was filtered so each sheet of film received only one of the primary colors. I am not sure how the prints were made but taking the pictures would have been a time consuming process. Because of the filtering, exposures would have been long. There may have been other methods but this seems to have been the most common.
Kodachrome came out in the mid 30's for movies and a couple of years later for still photos. Good film. My dad's Kodachromes from the 30's and 40's still look very good but the Ektachrome pictures from the 60's have faded badly.
I have never seen color WW I pictures. These are amazing.