I don't get it, what's it playing on?

Its name might be a play on "spellcheck". When I first read it quickly, my eyes bounced over the "h" and my brain filled in a "p".

Personally, I enjoy cleverness like this.

idk if it's a play on something -- but all of the things you mentioned above are still called a "shell", even if it's not the standard "sh" shell.

and, from parts of the source code, it appears it may work with ksh, zsh, etc. (i don't have a sample handy to try though).

not for me, I use rc