Since the Unix source code is widely available inside the sorts of large companies where many Linux contributors work, someone could have decided to take a shortcut when writing a complicated section of a program and just copied it from Unix.

<snip>

There is no formal mechanism for ensuring that the developers are not submitting code which does not belongs to them, only this unstructured peer review and that may not be enough.

Minor word change:

Since the Linux source code is widely available inside the sorts of large companies where many Unix developers work, someone could have decided to take a shortcut when writing a complicated section of a program and just copied it from Linux.

<snip>

There is no public mechanism for ensuring that the developers are not submitting code which does not belongs to them, only internal review and that may not be enough.

Glossing over this point also requires ignoring the fact that any code that is improperly placed in Linux can be easily identified and removed ... assuming the correct copyright holder actually wants it removed.