Of course moonscript would be optional. TIC like PICO-8 rely on LUA to work.
moonscript is a language that gets converted into LUA to work. (similar to typescript -> javascript or coffeescript -> javascript or Scala -> Java, ...). As such all functions defined in LUA can be used in moonscript (no additional implementation work on that side). However you cannot do an include("moonscript") because the syntax is different than LUA. We should define a comment tag to specify the use of moonscript instead.
Moonscript as it is more complex than LUA shouldn't be the default choice, I agree. But moonscript is also interresting for TIC (and PICO-8) because it requires less lines to do the same things as LUA. And since the screen size is limited, it would help a lot on big project for advanced programmers.