That was probably Stallman's doing. He wrote most of the LISP machine code.
That's nonsense. The Lisp Machine code was written by several people, some which had significant contributions.
After Symbolics formed, RMS did an amazing job at reimplementing many of the new features Moon and company were adding, so as to keep the MIT version of the system at rough parity with the Symbolics version. So his name belongs in the story, but not first.
https://danluu.com/symbolics-lisp-machines/
Also interesting bits about the fall of Symbolics, the AI winter, and why Lisp adoption waned as a result.