It doesn't grow from cons cells, which some people think is crucial for an authentic Lisp. They also tend to exclude Clojure.
I think maybe the Janet designers don't want to get bogged down in the language wars, and don't really care whether lispers enjoy their language or not.
A language with Lisp in the name and cons cells that a purist might argue isn't an authentic Lisp is Picolisp, e.g. because it has FEXPR:s rather than SEXPR:s and no macros. It's a great tool either way, easy to be very productive in when inventing ad hoc tools in a shell. The 'match function is quite different from e.g. the PEG:s in Janet but I really like it when doing something obtuse and gnarly with strings. Having a Prolog engine at hand is always good too.