Even assuming for a second that Google stores the location of every user (they didn’t store mine), how would they do contact tracing when they don’t have access to data on iOS devices? Even assuming they did, what key would they join on?

But no, a confident blanket assertion is made without providing any evidence.

They definitely store location for the majority of users who use Google Maps, their location history is both very accurate and opt-out buried in settings menus. I don’t have numbers on percentages given that Google likely doesn’t share this data, but I feel safe in assuming that less than 5% of Google Maps users have turned off the location history feature.

Many potentially dangereous contacts happen inside where positioning doesn't work. Maybe in a restaurant the location could be inferred because you enter and leave through the same door. But in the subway/metro/underground? Also every high-raise building would create lots of false possitives by people that did not get close.

Google location is still very good in buildings and subways because they wardrive open wifi routers to help triangulate you. So when I'm near a wifi router with a particular MAC address that google has seen before, they've got me.

Agree with you on high rises and false positives, but then again, if air is circulated in the building or everyone uses the same elevators, might be worth notifying them.

https://support.google.com/maps/answer/1725632?hl=en

Phone barometers can be used to get a pretty good idea which floor of a building you're on.

https://www.popsci.com/gadgets/article/2011-10/so-um-why-doe...

https://www.engadget.com/2012-09-06-stmicroelectronics-detai...