If you assume that the only way that the NSA could get a bug into apple products is by making a call to Apple execs and asking them to do it. What do you think of the possibility of them approaching someone who already codes at Apple, possibly someone who they've had previous contact with, or have leverage against that would guarantee that they won't go public about an (unsuccessful) approach from the NSA, and employing them to find places to insert deniable vulnerabilities into Apple products?

It's unlikely that Apple would agree to sabotage their own products in a way that will eventually be detected and affect their own credibility. That's restricting the argument that people are making to a straw man, though. Have spies ever worked that way? What people are mostly speculating about is whether this was an intentionally introduced vulnerability, not the mechanism by which that would come about.