Where I'd say it advances into breaking SBCL's jaw is that the runtime, interpreter, jitter, etc. are all much smaller than SBCL's runtime and compiler. If you're looking for a complete system, I'd say SBCL wins out obviously. You're talking a world-class REPL, debugger, a high quality stdlib, etc. All it's missing is a text editor like LispWorks (emacs and pretty much every other FOSS Lisp editor I've seen is a massive downgrade.) With that in mind, SBCL is not something you embed in an application written in another language. The holy grail is getting something as fast as SBCL, as flexible as SBCL, but as a 50k loc self-contained runtime. LuaJIT is the reigning heavyweight champ there, so having a Lisp-adjacent language like Fennel running atop it is a pretty damn compelling idea.

Interestingly with regards to text editors, Lua doesn't have that problem technically. Lite-XL is dangerously close to being zmacs/LispWorks for Lua. Poetically, just like Lua it's fairly bare bones and requires extension to be a decent IDE. But the underlying structure is absolutely fantastic, being based around a fairly cohesive object model rather than coats of paint over text buffers.