MacCarthy was definitely conducting Lisp as a language implementation project. He was just surprised that his specification of the evaluation could be literally translated to code and executed; at first he thought Russell misunderstood that the spec was just for people to read, not for the machine. Someone was supposed to read that and implement the behavior.
It's kind of hard to understand why MacCarthy was surprised. Anyone sitting down to follow the spec and make it work on the machine would soon realize that they are just mechanically converting the expressions to code. Like where the specification calls APPLY with certain arguments, your machine code has to obtain those argument values and call APPLY in the same way. It will soon be obvious that you're just hand-coding exactly what is written. People were already doing that. FORTRAN is "formula translator"; before Fortran, formulas had to be translated to code by hand. Russell was just "fortranning" the formulas in the Lisp specification, like you would arithmetic formulas.
It's kind of hard to understand why MacCarthy was surprised. Anyone sitting down to follow the spec and make it work on the machine would soon realize that they are just mechanically converting the expressions to code. Like where the specification calls APPLY with certain arguments, your machine code has to obtain those argument values and call APPLY in the same way. It will soon be obvious that you're just hand-coding exactly what is written. People were already doing that. FORTRAN is "formula translator"; before Fortran, formulas had to be translated to code by hand. Russell was just "fortranning" the formulas in the Lisp specification, like you would arithmetic formulas.