Rasbian is not a fork of "Linux". It is a distro that contains lots and lots and lots of open software. Including Linux. Just like Ubuntu and Alpine and Debian and Rocky and Neon and hundreds of other distros do.
That distro is fully open .
It includes an open, non-closed version of the Linux kernel.
You can, and people absolutely do, use Linux from the Linux foundation, on tons of ARM hardware, including Raspberry Pis and other ARM-based general purpose computers
Can I run desktop Ubuntu on my Motorola phone? No. But I can't run it on a Kankun smart plug either, and that runs Linux. Ubuntu wasn't designed to run on this hardware (though they did work a LOT on specific phone and tablet support). I also can't run it on a Milk-V Duo, but it is Risc-V and runs "Linux".
Do many phone manufacturers do other things to lock down phones? Yes, but that has nothing to do with being ARM-based. And I was responding specifically to the ARM claims.