But that is geared towards not spooking a live animal. Irrelevant when talking about overtaking cyclists (who also have to overtake horses, and slow down to do so)

Irrelevant? I'd call it almost exactly the same.

Whilst a human being may understand they're sharing the roads with motor vehicles it's still very easy to creep up on a cyclist as 1. they probably won't hear you till the last second due to wind noise and 2. when you combine that with passing too close it's very easy to see how a bad overtake might cause an incident.

You can always hear a car behind when on a bike
You can overtake a cyclist with a 30/40/50mph speed differential if you are 1.5m away and it will be 100% fine. You couldn't and wouldn't do that to a horse.

Overtaking a horse on a road is very different to overtaking a cyclist.

I agree with your comment "Overtaking a horse on a road is very different to overtaking a cyclist." but have to question whether you have ever ridden a bike.

I've ridden over 18,000km this year alone, so far.

So you average over 50 miles per day, every day ? 3 hours or so ?

I like a ride but not that much smile