Blue Cross rescue centres are testament to the fact that not all horse owners are "sentient" when it comes to providing a suitable diet for their individual horse.
Even here on HF, occasionally a member will post a picture of their horse, blissfully unaware that they are about to be slated for it being too thin or too fat (the latter happened earlier this week).
I would like to think that anyone who actively seeks this leaflet out has already acknowledged there is an issue and will be, as you say, sentient enough to decide whether or not the regime is suitable for their horse. This was just my personal experience as someone who attempted to make use of it. Others may find it extremely helpful.
I applaud the charity for shining a light on equine obesity.