Do you have a citation for this? In my experience, pairing a new key requires either providing a cryptographically signed certificate or having an existing paired key within range before a new key can be added.
the 90s era hondas up to about 2001 use various key-turn-rituals to enroll/program keys into the immobilizer, the later ones use the Honda HDS system which is just a specialty Toshiba/Panasonic ToughBook with an obd dongle and special software.[0]
I've enrolled keys myself for my 04 BMW with bootleg 'BMW MODIC' and 'BMW Rheingold' software packs pirated from The Pirate Bay.
You don't need existing keys for either system.
The trick (used to be) at the time that BMW keys were difficult to cut, and the key cutters were well controlled. This isn't the case any more, and in reality if a key was the deterrent you could always just program an immobilizer chip from another key, tape the key/chip to the column, and then use a pry bar and screwdriver to break the key tumbler and turn the switch without a key. This is neither rare nor hard to do -- and it used to be the defacto way to steal pre-immobilizer Hondas (breaking the column/tumbler, that is).
It was common enough that an in-joke at the Honda dealership I worked at was that a flathead screwdriver could be referred to as a 'lazy CRX key', a majority of those era cars encountered were so worn that a flat head would turn most of their tumblers by the time I got to work on them.
[0] : I was a Honda tech from 07ish to 09ish
So now I have an 80 and an 81 Accord... and I have also interchanged keys between them. The 80 doesn't open as easily, as I think it's less worn out. But there's practically no security on these old Hondas.
https://www.cbc.ca/news/canada/marketplace-car-thefts-1.6396...
I don't think it was limited to Chevies.
Right to repair has two sides of the coin.