I think it's more nuanced than that. It's true that their OS is closed so we really don't know. But we can make some assumptions from what we see. We do know that the FBI demanded they build in a backdoor to their encryption a few years ago and Apple refused, claiming that they don't have access and aren't going to rewrite their code to add a backdoor. We also know the FBI then either believed them or decided it wasn't worth trying to pressure them, so they hired a third party hacking company that claimed they knew a workaround that would give access to the info if the user hadn't enabled encryption, and it seems like their hack did work. A few weeks after that they was a story that the FBI was angry because Apple closed that workaround. But of course we can guess that behind the scenes the FBI and other government organizations are still working to gain access and may have gotten it. We might also assume that if the FBI did find another workaround to gain access to our data they wouldn't announce it to avoid Apple patching the hole.
Furthermore, a lot on the phone isn't encrypted at all. If you sync any data with Google or Dropbox, that is visible to anyone who knows how to look. And we know from news stories that both Google and Dropbox are not against selling our info. Email and SMS are visible to anyone. Apple Messanger is encrypted on both ends and again Apple claims not to have access. And again the government is pressuring Apple to open Messanger up, now opening an Anti-trust lawsuit to force them to open it up. That doesn't tell us if Apple is telling the truth about not having access, but it does suggest the gov believes them and so if taking a more forceful approach to get access.
We also have engineers who have quit Apple who make claims about the security. And there is the fact that more than any other company, Apple has staked their reputation on keeping our info private. They gain nothing by secretly going against that and they have everything to lose.
So it's yes and no. I don't know if we should assume the worst That's taking the extreme position that I argue against in my post. But we also shouldn't blindly trust and should watch to see what's going on. I also think it's probably best to avoid putting very sensitive info on our phone.