Proprietary makes it more vulnerable to hacking. That's not a positive. It's a massive negative. IPhone security has been through good internal design, walled garden application loading, and luck.
I think security is quite independent of being "proprietary". What counts is having developers who know what they are doing, who have the support of their management, and importantly how any compromises between security and convenience are handled.
A trivial case of security vs. convenience: Lots of things on my computers are protected by two factor authentication. To attack me, you need to steal my username, password, and my iPhone (unlocked). Without two factor authentication you would only need to steal my username and password. So it's more secure. It's also less convenient, because from time to time a need to take my phone out of my pocked, read a six digit code, and type it in somewhere. That's inconvenient. Your choice if you want convenience or security.
iPhone security against hackers (or governments) unlocking your code is quite good, because the guys implementing it are competent, and they have the support of their management. Sometimes security in five your old devices is not perfect, but most of the time these things are already fixed in newer devices. Of course when you buy an iPhone 8, nobody can predict what attacks hackers can come up with in the next five years - but you can be quite confident that whatever is used to breach an iPhone 8 will already be fixed on the iPhone 10 that is the latest best seller.
"Proprietary" doesn't help much, except you can expect anyone being incompetent to hide what they are doing. Apple has published a lot of information how their security works. And for various good reasons, a lot of it is of the type "even Apple itself couldn't break this. We designed it so that we can't break it". Again, that's security vs. convenience: If a relative dies while their phone is locked, you won't be able to unlock it and use it, and Apple won't be able to help you.
On the other hand, an open implementation doesn't mean it's safe. There was the whole OpenSSL debacle. I had the joy and fun to look at some of the source code, and it was just bad. It was impossible to figure out what anything in that code was doing. I've rarely been swearing that much looking at anyone's source code. There were some big bugs that could be and were exploited, but they were very, very hard to find. (To be fair, the main author apparently wrote it because he wanted to teach himself C++ programming, and though something like OpenSSL would be a worthwhile practice project. So we can't really blame him. We can blame the community for living with this mess).
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Also you need to remember, Apples iCloud has been hacked a few times, as reported in the media, so it's not as secure as you claim because that's where the iPhone automatically backs up to. I don't give Apple kudos for that!
Ok, give us some believable evidence that iCloud was _ever_ hacked. There is none. As usually, you are making it up.
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If you have an iPhone that isn't jailbroken and one that is would you say without a shadow of a doubt they are both equally vulnerable to an attack be it in or out of the users possession (assuming they were both locked equally)?
It depends. First, during the jailbreak, you allowed the code created by some third party company to take control of your code. That's in my opinion the biggest vulnerability: Whoever created that jailbreak could have accepted a million dollars (or a get-out-of-jail-free card) from the NSA, and now the NSA has control of your phone.
Second, there have been jailbreaks that opened stupid security holes on iPhones. One that allowed remote login with a known default password; that was quite awfully bad. I haven't heard such things recently.
Third, you can download apps from third party sites. That obviously opens any amount of possible vulnerabilities. Especially if a jailbreak destroys the separation between apps (don't know if it can).
The first hurdle to attack your iPhone is unlocking it. This ability is nowadays hidden _very_ deeply inside the iPhone hardware. It might be impossible for a jailbreak to even affect this; in this case jailbreaking would be as safe against unlocking the phone as a non-jailbroken phone
