There’s not much argument. The rule is clear. A player - when snookered on a colour - must declare what ball he intends to hit. If he doesn’t, the ref will call foul and award 7 points to his opponent. Crystal clear.
The problem with your position is you are retrospectively judging a near 40 year old clip by modern officiating protocol and completely missing the context.
That situation could almost certainly not happen these days. No referee would simply let Thorburn get down and play the shot without checking to see what colour he is nominating. Hence the incident with ronnie and terry camilleri i referenced above. The refs will remind the player if needs be, they won’t just stand idly by, waiting to shout FOUL as soon as the shot is taken. It’s not how the game works.
I’m not 100% on this but i would still be fairly certain that this protocol was developed and initiated by referees in response to incidents like the thorburn one and that’s how it evolved over the years, to ensure those kind of arguments don’t break out anymore. It’s not just the players responsibility, ref has a role to play too.