This is just wrong. Apple's public position could differ from its actual one. And Hiding a backdoor (or hiding with the deniability of an innocent bug) in open source is possible. "we'd definitely know about it" is not how security works.
I would certainly consider it unlikely enough that any suggestion to the contrary, without evidence, is just a conspiracy theory.
Yes, but extremely hard is not the same as impossible. How many security 'bugs' can we be sure were unintentional?
This is not just semantics. Complacency can come from over-confidence.