That sentence refers to the interpreter as an evaluator for Lisp, where McCarthy had the idea and Steve Russell suggested that it could be implemented - and the

That sentence refers to the interpreter as an evaluator for Lisp, where McCarthy had the idea and Steve Russell suggested that it could be implemented - and then also did it. Before, code was manually translated to an actual implementation on the machine. It seems also that it took time to get details and the semantics right - especially the problem of of lexical binding (the FUNARG problem) took more than a decade to fix.