Why would anybody ever think that the absence of an exposure notification (via this app) would mean that one wasn't exposed to the virus? That would be like thinking that after being out on the road: 'Nobody honked at me today, I must have followed all traffic rules perfectly'. Everybody knows that this app is just one puzzle piece that can help to quarantine (and potentially treat) some new infections (via prompting people to quarantine and get tested). Everybody knows that the app doesn't have 100% uptake, that infection is a stochastic process, that there will be many false positives and false negatives. It's like wearing reflective clothing as a pedestrian or cyclist, nobody would think that that saves them from being hit by a car. Everybody knows that if the app reports an encounter, one should take action and if it doesn't one should behave the same way as one did before installing the app. The app complements traditional contact tracing. Nobody thinks that if no official contact tracer has reached out to me today, I am at zero risk of having contracted the virus.
Everybody knows that they should always behave as if they were infected (without knowing it) and try to minimize the risk of infecting others (by wearing a mask, keeping a distance, avoiding unventilated interiors, avoiding crowds, etc.). The app reporting no known encounters doesn't change squat in that regard. Nobody would think that they don't have to wash their hands just because the app reports no known encounters. You wash your hands regardless.
Why would it be impossible to be useful? It is registering plenty of encounters every day. If just one of those encounters is reported as positive, the app will have generated a testing candidate. That is useful.
If 50% of all people have it installed than 50% of all receiving a positive test result can trigger exposure notifications, those will be useful only to the 50% that have installed the app, thus achieving 25% of the exposure notifications compared to a situation where 100% have the app installed. But for you apparently anything less than 100% success (in achieving the theoretical maximum potential) is not useful.
And you repeating the word 'dangerous' over and over doesn't make it so. You have provided zero explanation what could be dangerous with this app. The only thing dangerous here is you spreading lies about the app.