I follow your line of thinking, but what's missing from it is that repl.it isn't unique at it's core. There are a ton of "multi language repls", many of which existed before repl.it, before Amjad's jsConsole, etc.

What repl.it is doing that is unique is the amount of investment into all of the "extra" stuff around it.

The "number of languages" thing isn't secret sauce either...clearly once you have the base repl working, iterating more languages is pretty straightforward.