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.