I don't agree it is clear. The onboard F menu says you can dine a la carte, which to me means you can eat what you want from the menu, but not necessarily when you want. I'm not aware of any BA F menu I've seen in the last several years which is clear that it is dine on demand when it comes to timing (there is a brief mention on BA.com in the travel classes section, but nothing onboard).

IME the crew rarely ask you when you would like to eat, the assumption is that you will eat at their preferred time. I don't think passengers inexperienced with BA F are necessarily aware it is dine on demand, and it would be so easy to ensure everyone knows.