Did you even read all that tosh?
It really doesn't say what you say it does. He says he doesn't recommend that a "beginner" does that.
There are many others like DR, Newtone, Thomastik who all make roundcore strings and are happy that they will be cut, and they give instructions as to how to do it properly.
Maybe he's only referring to GHS products, but it is certainly not true to say that roundcore strings should never be cut. They just have to be cut properly.
https://www.drstrings.com/proper-string-installation