the whole “one tone sounding like a road vehicle” doesn’t really wash with me, because no matter the sound, most people will hear a train horn and look up, no matter what it might be. I mean, it’s quite loud and distinctive after all.
You may think that, I disagree. There are numerous types of rolling stock, and not all have horns that sound the same.
Similarly, there are numerous types of road vehicles, and again, a lot of them sound different.
So if you are not near the source of the horn sound, it is not always clear if the sound of the horn you hear is that of a train or a road vehicle. Or indeed, something else (such as a boat or a ship).
Structures like cuttings, embankments, large buildings, etc. can all make it harder to distinguish where and what has just sounded a horn.
Whereas, when a train sounds a two tone horn, because that type of sound is unusual for any other type of horn equipped source, people easily associate the sound with that of a train.
It's interesting to consider the use of twin tones in excess of the rule book requirements but in accordance with union instructions, because I think it was a legal challenge that brought about the instruction to use a single tone - presumably those who use twin tones (rightly, in my opinion) might be vulnerable to legal challenges, or their TOC or union might be.
As the two tone horn is permitted for other uses, I don’t think it’s likely that there would be any legal case that would be viable. That does not mean that the railway did change the rules to try to minimise the impact of noise on it’s neighbours.
Network Rail makes it very clear in its own internal documents that both employees and others working on the railway should be considerate with regards to noise levels.
There are many areas of the rule book that I take issue with. The old BR rule book was not that good. But since the RSSB took control, it’s most definitely got worse. Ever expanding, getting more complex, but good practice and any helpful parts getting removed. Hence, some now consider it more useful as a paperweight.