Re: I did not know that
You have always been able to a secondary processor alongside the ARM core.
So the 'prohibition' consisted of two parts: you couldn't modify the ARM core, and the ARM core provided no means of adding instructions.
Also, FWIW, the quotation in the article is that you can use RTL -- register transfer level -- to add your instructions to the new interface. That means you'll not necessarily have to use "logic gates" as the author has suggested.