I have done this. iOS may be closed source, but firmwares are no longer encrypted, so you can reverse engineer anything that you are curious about.

Sure, you can see the code / source / raw binary. But actually analysing and verifying every part for security or privacy concerns - bearing in mind you have to consider all the possible interactions this code can generate - and that's before you even get to the hardware interactions - is still beyond 99% of developers, never mind normal people.

I was speaking to "There is no privacy if you can't inspect it and verify that it's upholding its promises" actually, and I probably agree with you, as your point applies just as much to open source.