"Essential" implies that one cannot be competitively successful without strength training. The facts are years and tens of thousands of triathletes that have successfully prepared and competed in triathlon without a single day of strength training. Multitudes have taken the podium at various levels without strength training. Kind of hard to buy that it is essential when so many have done without it.

Though strength training is and has been my specialty for over 30 years I am not quite sold that it is "essential" for the triathlete. I have no dog in this fight so I will not continue on. I gave the OP a decent source of information to look at for free.