Warm-up and Warm Down

I am new to tri-training and I have a question regarding warm-up and cool down in swimming. I swim intervals three times a week with a fellow (49 years old) who has been competing for many years. He stresses warm-up including easy swimming, drill and kick and a thorough cool down. Warm-up is usually around 800m and cool down 300 - 400m of easy swimming or drilling down. Total workout is 3000 - 3500 m. Another fellow who swims at the same time (but not with us) ALWAYS starts his swim with a 400 build and goes very hard after the first 100 - 150 m. He then does a main set of intervals totalling between 800 and 1000 m and his warm down is always 50m. He says 400 build as warm up and 50 m cool down is more than adequate. I may be new to training but this seems to be inadequate for warm-up (if going hard after 100 - 150m could be called warm-up) and warm down. This seems to contradict all principles of training and athletic performance that I have read about. Comments? Thanks