While that might be an argument in general, the protocol Google and apple are working in is cryptographically secure. They provably can't back out data about you.
I believe your claim is false.
That would be true even if Apple suddenly decided "oh these folks on HN want to see the code for that tracing component, better open source our complete OS". I've seen assurances that parts of this will be open sourced as far as they can but I honestly don't see how that matters. You have, as is a general tradition, a description of what this system does cryptographically. The rest is a matter of reverse engineering that hardly benefits from having the code. It's not like Play store updates or whatever Apple uses to push this are reproducible builds the end user can verify. And you always have the closed source blob of whatever BTLE chipset your device uses which you can mistrust.
edit: Let me qualify "but I honestly don't see how that matters" for the question of trust in what runs on your phone here. If you read a dig at OSS into this you're generalizing far over what I'm addressing here.
I have compiled some of the apps on my phone myself so I know that the result does in fact come from the same source code. That may not work in the case of iOS devices, though.
> And you always have the closed source blob of whatever BTLE chipset your device uses which you can mistrust.
That is a good point and it'd be great if that chipset could also be open sourced but one step at a time.
> That would be true even if Apple suddenly decided "oh these folks on HN want to see the code for that tracing component, better open source our complete OS".
You are correct if it was simply open sourcing the OS. If it was possible to compile and install the OS ourselves, that would completely change the game.