the OS was not the comparison, but the hardware it runs on (just as @Freefall said) but also you seem to be wrong with your other assumption:

And both those devices are tied to a specific OS.

Which seems not to be the case as install instructions for another OS can be found here (i didn't try it though) for the mentioned device:

https://wiki.lineageos.org/devices/pdx215/

lineage os still is an "android", but another vendor with clearly different approach than the original firmware and what hinders you from writing bsd drivers and compiling a bsd kernel for it instead? So i count the Xperia 1 III as NOT bound to any OS or OS vendor.

But despite the way longer possible support/security, freedom of choice and endless other possibilities that often come along with free OS choice, this pure and great advantages weren't even mentioned there, thus it wasnt an OS comparison as it also wasn't a bound-to-an-OS vs. absentness of vendor-lock-in-limitation-jungle comparison.