I think it's that AtomVM is erlang specific, while nerves is elixir and erlang. I could be very wrong, but that's my initial read from the docs
I see plenty of references to Elixir in the docs, and this official AtomVM project to provide some tooling to target AtomVM from Elixir
Will that extra 512kB be enough for fun stuff?
I'm not a BEAM developer, but a quick search found this [1] which was very informative. My classification would be "quite fluffy", but that's of course highly subjective and the language/platform has quite a lot of features that it needs to support so I wasn't expecting CHIP-8, exactly.