Did you listen to the first comment at the very beginning?
"Swimming fast is the best thing you can do for good swimming mechanics". That's where training comes in with frequency."
Also listen from the first words that come b/f that statement. Swimming slow and easy all the time does not fill out the equation of speed. At some point if speed is the goal you gotta get out of the easy mode. There is a lot of great stuff in there that is dead on accurate. 3:50 is the exact reason why a lot of adult swimmers are so slow and never get substantially faster.