That's a pretty cool project. Sure you CAN do it in any language (as another commenter points out, you can do it in C too). Anything that's turing-complete can do it. You could also use C to parse ASCII art and have that create a TCP implementation. It's not the natural way you'd do it in that language, though.

lapp has been around for four years; docopt is (looking at git commits) less than a year old. It strikes me as a natural way to do things in Lua, which is why (I suspect) it was done in Lua earlier.