Re: One would have throught...
It's worth observing that perfection isn't required. This is about risk reduction, which is a game of percentages. The higher the percentage of coverage, the better the risk reduction.
You're right insofar that waiting on a perfect app would get us killed, but (a) it is but one of the measures (you still need to keep distancing and where possible masking going) and (b) it isn't required. On its own it would not be feasible anyway because it relies on a fundamental assumption that everyone has a phone that can run this app, and that they keep it properly charged (aka ignoring poorer people - again).