This is the latest I read:
https://pix11.com/...tpone-nyc-triathlon/ Based on this, I would say it's a stretch to say "de Blasio canceled the race".
Furthermore, I think it's actually a prudent decision by the organizer to cancel the event. This is not about just the health of the athletes, but also all New Yorkers when the city's EMT services will be impacted if there are a large number of athletes needing emergency care. Based on what I've heard, NYC Triathlon is one of these destination events that attract a lot of novice or non-regular triathletes, so they may be particularly susceptible to the heat and poor air quality.