No interpreters used, mostly SBCL and CCL. Folks over at Grammarly have a blog post which mentions few reasons for the combo http://tech.grammarly.com/blog/posts/Running-Lisp-in-Product...

I write portable code and use libraries which abstract away implementation specific stuff so it will be easy to run on any other conforming implementation should the need arise.

The community is quite responsible when it comes to portability and utility libraries which help with it, but on top of that Quicklisp maintainer tests library compilation on most major implementations and reports bugs.