I don't think anyone is calling testing a preventative, it's an identification tool to isolate individuals or re-schedule games. And I think its very commendable the NFL is doing daily testing and re-testing. This is a Titans issue more than an NFL issue, and somewhere the Titans fucked up the guidelines.
The incubation is how long it takes before someone shows symptoms, so two separate things there. I've always used the 2 week window if I have to venture out to a public place. I had to do a training session a month ago in a pretty crowded computer lab, and I didn't have any symptoms. Could I have been asymptomatic, sure, but a test would've revealed it.