1. Seems kinda hand-wavy to me, so I’ll boil this down to lower bloat (i.e. lower disk and mem usage by the OS)

They pretty clearly say what they mean by that though, unless you only read the headers and not the actual text

2. This is very much YMMV, and for Steam Deck specifically, it’s comparing a tuned the system to an OOTB experience; surely other handhelds tune their systems too

They absolutely don't, is the thing, and the Windows ones largely can't in the same way that the Deck can, because they can't change how Windows works beyond the surface, meanwhile Valve is able to write software for Linux like Gamescope, an entire lightweight compositor that lets them have full control over how games are displayed and means they don't need to have a full desktop environment running, and they directly contribute to and fund development for open source system components (like the KDE Plasma desktop environment that's used in Desktop Mode) in a way that would be impossible for similar things on Windows

Valve even has their own custom patched version of the Linux kernel in SteamOS, you can't do anything remotely like that on Windows

3. I’m pretty sure this is true for other handhelds, but I haven’t used them personally so I don’t know

You can't avoid having using the desktop eventually on Windows on a handheld, and it's always running the background, even if you boot into Big Picture

Even if you're always running games from Big Picture or whatever, you still need to use the desktop for updates, as well as any settings and functionality that can't be accessed from Big Picture on Windows (like dealing with Bluetooth devices), as opposed to SteamOS where all of it can be handled directly in gaming mode without a desktop even running

4. This seems very solvable, and not an inherent Windows issue; large enterprises manage drivers and whatnot centrally, surely a handheld can too

ASUS already has a solution, like the article mentions, but it can't be nearly as seamless as SteamOS where they can just push a single system update image that includes everything, and it's applied all in one go directly from gaming mode

There's also additional benefits SteamOS can have with its update system that Windows can't have, like how it has an A/B partition system similar to Android so that a broken update only breaks one partition and it can switch to the other one when that happens, which especially helps if something like a power interruption happens during an update and it doesn't complete properly (meanwhile on Windows it can be pretty hard to recover from something like that)

5. Surely this is true for Windows devices, no? I’m guessing more people are comfortable customizing Windows handheld PCs vs the Steam Deck simply because more people are familiar with customizing Windows than Linux

You absolutely cannot modify Windows nearly as deeply as you can with Linux, and attempting to make any serious changes requires hacky solutions that Microsoft can just break in the next update anyway

Like, you can change almost every single component of a Linux distro, you can rewrite components directly since they're open source, and there are usually multiple options to pick from for any given piece of system software, such as the entire desktop environment, or the audio system, or even the kernel itself