Note that this 70% does not refer specifically to patients who were at the game. Just a "majority" and "many" were at the game. The article also never states the actual number of patients suffering from frostbite. Just 70% of some unknown total.
You'll also notice the URL says "70% of Chiefs fan" yet the actual title in the article is "Many Chiefs fans who suffered frostbite at bitter cold playoff game need amputations". Wonder why there's a difference
From another article on the subject:
https://www.npr.org/2024/01/17/1225209300/chiefs-dolphins-hypothermia-cold?darkschemeovr=1
Out of a stated attendance of 71,492 people, at most 3 had something (a foot? a little off the end of the pinkie toe?) amputated. Not quite as sensational a number as "70%"