So a security threat gets reported to this bug bounty team. They are able to reproduce and confirm. The bug is in some deep, crusty part of the kernel; the code for which isn't available to this team, because Silos.
The team who does have access to this Silo is tracked down. It gets processed into a ticket. Maybe it gets done, maybe it doesn't. Their backlog is already maxed out, as is everyone's.
The security team says "we've done all we can".
This is not a matter of "lol just hire a team". You need leadership aligned, so the manager's manager can see their task list, or open security issues, and say "what the fuck, why are you not prioritizing this".
That's not Apple. Apple is product-driven. They actually, legitimately don't care about Privacy and Security. Their manager-managers get mad when products aren't delivered on time. They may also push-back on purposeful anti-privacy decisions. Its not in their culture to push back on Security issues, or latent Privacy issues resulting from those Security issues.
"Just tear down the silos" > Working for Apple is a cult. The silos are a part of the cult; left-over from one of the worst corporate leaders of all time, Jobs. Try telling a cult member their beliefs are false.
"Grant the security team access to everything" > And, I guess, also hire the smartest humans to ever exist on the planet to be able to implement security improvements across thousands of repositories in dozens of programming languages, billions of lines of code? And, even if they could, push those changes through a drive-by review and deployment with a team on the other side of the planet you've never met? You, coming into their house, and effectively saying "y'all are incompetent, this is insecure, merge this" (jeeze ok, we'll get to it, maybe in... iOS 18)