Forgive me for being a naif, but can someone explain why the derivation presented in the old Mechanical Universe video from the 80's is problematic? I'm talkin

Forgive me for being a naif, but can someone explain why the derivation presented in the old Mechanical Universe video from the 80's is problematic? I'm talking about the part that starts around 19 minutes in. https://www.youtube.com/watch?v=SS4tcajTsW8 https://www.youtube.com/watch?v=SS4tcajTsW8