Oxford Medical model says defeating covid19 requires digital contact tracing app

'Our mathematical modelling suggests that traditional public health contact tracing methods are too slow to keep up with this virus.'

'Our research of early data from other countries shows that patient histories are incomplete - we don’t know the details of the person we sat next to on the bus. We need an instantaneous and anonymous digital solution to confirm our person-to-person contact history.'

here is the announcement - http://www.ox.ac.uk/news/2020-03-17-infectious-disease-experts-provide-evidence-coronavirus-mobile-app-instant-contact

Here is the proposal - https://045.medsci.ox.ac.uk/mobile-app

Couldn't FB and Goog. just do this with a switch? I understand the privacy implications, but their model says we can't do this effectively without it. We know S. Korea has this and that it helps prevent the need for full lockdowns. How could we make this work?