Mark Austin: you are a bit confused. Of course Brasser did not change the nature of the music by writing it down, and of course the definition(as pointed out earlier more than once) nowhere says transmission has to be only oral. That would be nonsensical, which is why it doesn't say it.
In practise, by the way, within the Copper family, I would say transmission has been oral as far as I know. The songs are learned by singing. The books are used as an aide-memoire.