You're mixing up software and hardware. I have no strong opinion on the security of Apple's (macOS) software from a user perspective. It's a proprietary OS. It gets some things right and some things wrong. There have been privacy concerns (e.g. the CSAM mess). I use it for browsing the web sometimes, but I wouldn't make it my main OS.
But Apple deeply cares about platform security, and notoriously, iOS devices are some of the most secure consumer devices available. This isn't marketing bullshit - their designs are actually that good, which is something I can say as a security professional. You may or may not agree with their motivation, which ostensibly includes both customer security and keeping an iron grip on their iOS devices. But the end result is they have built excellent silicon designs with advanced security features and a very security-conscious architecture throughout. The same stuff that makes it hard to jailbreak iPhones. And so now that they stuck them in Macs and unlocked the bootloader, would I buy one? Of course. And put Linux on it. And so should you*, if you care about security. There really isn't anything else done nearly as well as these things, at least not at a performance level we'd consider decent in 2021.
Yes, it might surprise you coming from Apple, but it makes sense because they did this for their own benefit. It just so happens that their motives end up with a result that aligns with what I want. And so I'll take it, thanks.
I still won't use an iPhone, though.
* Okay, maybe wait until we're done porting things and it runs well.