It's a computer. You install the contemporary, open PC platform's software on it. There is nothing "custom" about that deployment. SteamOS itself is a version of Linux, based now on Arch, still including a complete desktop to boot up. It's generally the same as installing Steam on Windows and booting with Big Picture mode.
What is a console? A dedicated gaming device. OK. Bullshit, but fine. Can you install anything you want on it? No. It's dedicated to gaming and a controlled ecosystem. What is a PC? It's a standard pile of different hardware options that all prop up software people are free to do anything on. All the utility that comes with that is on the Steam Deck, and not the Nintendo Switch by a comparison.
Is an OEM "Gaming PC" considered a console if it's in a small box? No. It can do the things a console can, and outperform them in other use cases. It lets you install drivers and other operating systems that typically give you full access to your hardware.
Consoles don't do that. Even if you want to jerk off about PS3 allowing you to boot another OS or how the Xbox gives you Dev mode. They are not granting full access to the hardware and allowing you to run any code or OS you want.
Valve runs a dedicated gaming platform, Steam. Steam is just a thing you can run on the Steam Deck, and you're free to get rid of it. How? Because it's a PC.
This thread is fucking stupid.