From OP in the Reddit thread:

Got five minutes of No Man’s Sky in, then I installed the update the machine had available and it bricked itself. If you’re still in the queue, look on the bright side: they’re presumably going to iron this crap out. Edit: To be more accurate, it’s giving the error light code for GPU failure.

Anticlimactic and somewhat embarrassing update: as some people suggested, I left it unplugged for about half an hour last night and then tried plugging it back in… and it didn’t work. So I left it unplugged for a couple of hours and then tried it again before bed… and it didn’t work. Same error light despite multiple power-cycling attempts. So I left it unplugged overnight and plugged it back in today to try some of the BIOS stuff that other people suggested… and it booted up immediately without issue.

I feel stupid about even posting this now, especially since it blew up a bit, but I was tired and irritable after a long day of work, and an ominous GPU error code wasn’t exactly the seamless plug-and-play experience I had hoped for. But I guess if anyone encounters the same error, don’t panic like I did, just let it sit for a few hours and it will somehow sort itself out.

It sounds like a firmware issue that broke display out. Perhaps Valve pushed a fix while it was “on” overnight, or OP reset something in the BIOS.

What I’m getting at is the specter this raises: The Steam machine seemingly relies on Valve for firmware support here, I think. Not AMD directly, like with the normal Linux firmware shipping.

That’s an interesting can of worms.

It’s not bad or anything, I’d trust Valve more than HP, Sony, or past OEMs who were notorious for wonky “custom” hardware support. And I suppose their track record with the Deck speaks for itself. But still, that’s something I’d keep in mind before buying one of these things.