Also with an approachable kernel space that mere mortals not only have control over, but can extend. macOS is among the OSes that have almost eliminated access, requiring all loadable kernel extensions to be signed by Apple. Building them yourself isn't as easy as it had been only a few years ago, even with a dev account. (newer macOS even prefers a userspace solution with better security, but i digress)
Linux, on the other hand, rather straight forward to compile and load. And for hardware tasks, a Pi actually does provide a lot of surface area for a hardware engineer to work with.
There isn't as much performance as mainstream machines, but there's plenty enough. Heck, there is H.264 acceleration in the Pi, and it's exposed by the V4L2 apis for any userspace program to use (ffmpeg, etc).
I think "open" is the word to describe this.
I think "security through obscurity" is the word.
Apple has to tightened macOS down, because fucking subhuman cyberterrorists relentlessly and viciously poke and prod at every conceivable framework and OS command to the point where it gets harder and harder to have a system that harkens back to a time where you could just depend on simple Unix permissions to keep curious paws out of dangerous places in the OS.
Unfortunately, macOS has gotten to the level of popularity that it just can't be as Dev-friendly as it used to be, and remain safe and secure for Normies.
Raspian doesn't have that problem. It's still living in a world where there is simply no concerted interest in wreaking havoc on a semi-nanosecond basis on every single line of code in the OS, nor a wide swath of hugely-complex Applications, like web browsers, that a large-enough number of Pi Users can be almost guaranteed of Running.
Ken is just getting nostalgic for the good old days. When most malicious hacking was at the level of making everyone's terminal randomly output "GIVE ME A COOKIE".
Those days are long past; and Ken is just pining for them.
I suspect he'll be back, as he realizes that Raspbian just doesn't cut it as a daily driver.
I think you mistake the security model that is working against the developer, which these days involves more encryption than obscurity. The public apis, which may be more restrictive than linux, are documented and a lot can be done. But even with this documentation, installing your own code in the kernel is encumbered by this hurdle of getting Apple to sign off (notarization will be required soon). That's not obscurity, thats encryption with a dose of gate-keeping while we're at it.
Raspian's ability to get around the security problem by your statement is actually still obscurity, just a different kind (synonym abuse here, since it's not hidden and "obsucred" but instead more "obscure", different, and uninteresting as you appear to describe)
Using a Pi as a daily driver is viable, depending on what you want. For a system that will make you aware of every task it's doing, in which case you can't easily sneak in an unwanted supplier-provided service if you want to, it's actually kinda great. Bare bones features and performance, which even phones aren't these days. If you pine for 15 years ago before your device/computer was part of the System, performance to match isn't actually as bad as it sounds. And the only modern operating system you'll find for such a platform that isn't horrendous to use and support? Linux.