And what is supported hardware here? What even is "support"?

I'm writing this from Purism Librem 14, which works flawlessly, including suspend. There's also System76, Framework and more. See also: https://news.ycombinator.com/item?id=32964519.

As far as I can tell, Chromebooks are the only truly supported GNU/Linux laptops.

System76 is my go-to. There are others. You can even get some major vendors (Dell, Lenovo) to ship with Linux preinstalled, though I don't know if the firmware or chips diverge from the Windows variants.

Basically any thinkpad