Writing Emacs, be it in C or whatever, is hardly a reason to not call Gosling an expert in Emacs, quite the contrary! But you might argue his approach to Emacs

Writing Emacs, be it in C or whatever, is hardly a reason to not call Gosling an expert in Emacs, quite the contrary! But you might argue his approach to Emacs is not idiomatic or not consistent with the community and so on, if there is evidence for that.