When one of those dialects is completely source-incompatible with every other dialect (the rest of them share non-trivial programs freely), and doesn't share the same core features as every other dialect (like list structure), then why is it still a dialect and not a different language? Calling all the other Lisps that share core features and run the same code different versions of one language makes sense to me; I don't see the point in applying it to any language that borrowed a couple ideas.
If I have a large codebase written in (any) Lisp, will porting it to Clojure be significantly easier than porting it to Javascript or Ruby or any other modernish language? Not really; it's a complete rewrite either way. That sounds to me like it might just be a different programming language.