Very nice, but how will it differentiate between close contacts and contacts trough walls, plexiglass, curtains etc when one gets infected? Or how will it calibrate individual devices to compensate for differences. Heck, if you wear it in a purse or in a pants pocket makes a rather large difference.
And we don’t know how long/close the proximity needs to be at what point during the infectious periode before. And the difference between sneezing/coughing or just breathing is huge.
I really would want this to work, but even besides security there are so many hurdles to tackle that it might never be useful.