Awkwardly, what fixed it for me...
download hekate, move the hekate_ctcaer_6.0.7.bin to the atmosphere folder on the RCMloader, rename to payload.bin so it matches the launch dot bat file, plug it in, and it immediately started booting. And the switch lives again!
EDIT Nevermind, I do get it to boot, but it still gives me an "atmosphere encountered an error". Checking again, it's the same error. It won't let me go any further. Pressing the pwoer button at this screen, it reboots, but just goes back to it again and again. Pressing the volume button now, it doesn't boot the Hekate portion; just tries to boot atmosphere again, now.
And it's showing the same 2123-0011 error.