Speaking as a front-line health care worker who does know what the app claims to do and how it claims to do it, I'm prepared to recognise its imperfections and say to everyone in this country who can: download it. Use it.
Australia is tantalisingly close to completely suppressing this thing. Yes, the epidemiology that the app is based on is probabilistic, but, guess what: transmission of the virus is probabilistic over a population.
And if, $deity forbid, clusters start popping up again, then we need to put data into the hands of public health physicians quickly. Contact tracing stops small clusters becoming big ones. That is the sole function of the app.