Here is the text in the laws of the game.
Being in possession is defined as 'holding or carrying the ball.'
A knock-on is 'to knock the ball towards the opponent's dead ball line with hand or arm.'
And 'A player shall be penalised if they deliberately knock on or pass forward.'
So, you can knock-on when you are not officially in possession (we all see this in almost every game).
And you can (according to the laws) be penalised for deliberately knocking-on... whether you are the player in possession or not is not mentioned in the laws.
And I don't buy this 'everyone knows' part. 'Trust me, everyone knows' is no way to codify the laws of a sport.
Anyway, I don't care if deliberate knock-on's are penalised or not.
But
They do happen and by the laws of the game, as they are written, they should be penalised.