> And there is no undefined behavior at all.

Well, there is. Violating type declarations is UB in Common Lisp, the only thing the standard says about it is "the consequences are undefined". In SBCL, an error will be signalled, but there's no guarantees about what other compilers might do. This could show up if a Lisp programmer who primarily used SBCL was relying on the type system to do some of their input validation for them when portable code would have to actually check it (ie, in their head they might think their program will raise an error if you pass it garbage, when really it just invokes UB if you pass it garbage). I still don't think it's that big of an issue, and mentioning it alongside benchmarks certainly indicates some misunderstanding, but it's not completely baseless.