I honestly didn't see the same thing as you, my understanding is that using the Lagrange Interpolation is the "clever" choice, but when you add more points, you'll end up with a higher order polynomial and widely different curve which is probably not what you wanted in the beginning.

I read the piece more as a reminder that most specs are usually incomplete/too specific/not conveying what is actually needed, and a warning against cornering oneself with a too clever solution. I also didn't see any big issues with their stance on FP, except maybe that they dismissed the Monoid solution that someone posted down in the comments that looks both robust and readable.