I'm a tad annoyed that it apparently breaks my space bar - arrow keys and pgup/pgdwn work, but space does nothing.

Google's fault. Thanks for letting me know, though.

Anyway, my basic reaction is that you give no interesting reasons for preferring a planetarium over a simulation

Right—the argument is pretty modest. It's mostly just that the planetarium hypothesis is on par with other hypotheses like the simulation argument.

I also don't understand how such an entity would even build a planetarium in the first place.

Yeah, I left this to "a wizard did it"—if you accept simulation, then you can mix and match bigger and smaller planetariums around your brain or around the solar system to pose various physical problems. The planetarium hypothesis is sort of continuous with the simulation hypothesis if you like simulationistic assumptions. [ETA: And I didn't address any of those problems at any scale, because there's a problem for each scale. Factor your intuitions about the improbability of actually engineering a planetarium into your a posteriori estimate, to get a custom fit probability.]