Its probably more likely that those people got COVID from somewhere off the golf course
Players will get COVID-19, inevitable, but that doesn't mean they can't play. The goal here is not "no player test positive the whole year", impossible. The key to being able to play the season is frequent testing and isolation. I heard they are planning on doing mouth swabs daily and nasal tests 3x a week. Once a player tests positive they go into isolation. That will help prevent the spread, plus with masking when they are inside in close proximity.
If they are testing all the players, coaches, refs before the games, its unlikely that it will spread during a game. What makes this virus tough is the possibility of asymptomatic spread, but if they are testing frequently that mitigates that risk.
My guess is the NFL will allow positive test players in isolation to be exempt from the 53 man roster until they test negative