• Ordinary programming: 5 person-years by two PhD students; 25,000 lines of C++.
• Symbolic Programming: 200 lines, a few days.
Where do you even begin decompressing a claim like this?
25k lines in fact could be considered small for that sort of C++ engine.
But it would have been reasonable at the time to explore the type system around such a thing and produce a PhD (or a couple of PhDs) from it over a period of a few years.
Of course even at that time there was prior art in the Lisp world, but after the AI winter 1980s no students were going to work in Lisp. C++ was where the jobs were going to be.
The modern world was very much formed by the 90s. Mathematica, Axiom, Maple, etc were all mature software written a decade or more ago. What people were doing wasn't writing a CAS from scratch but writing one that could slot into whatever program they needed at the time, e.g. chaotic simulation of the solar system: https://en.wikipedia.org/wiki/Structure_and_Interpretation_o...
It's got numerous limitations but it is definitely a small program.
Start writing the code and see where it takes you. Having done that type of work in grad school I moved from C++ to Guile for exploratory work for that very reason.
I then rewrote the slow bits in C as custom functions much like how you would in Python.
That would make the cpp loc =200+lisp implementation loc