"without ubiquitous, cheap, fast, and accurate testing, you can't confirm the app's diagnosis. So the alert is useless."
was turned into medical professionals asking someone about contacts, a la
"Someone you report you had coffee with, but who doesn't get the disease, is a false positive"
to show that getting asked about contacts by medical professionals isn't better than a contact tracing app, which is a straw man, i.e. Schneier didn't ask for medical professionals to ask those questions, he said those questions are useless without tests.
The dichotomy isn't between the app or a medical professional guessing, based on contacts, whether someone might be infected and whether they should self-quarantine. The dichotomy is between guessing infection based on contacts and testing for infection.