Yours is a British perspective, we aren't as tuned into dog-whistling to the extent that Americans are. They've been subjected to it for much longer than we have, and learned to spot it more readily and "call it out". For instance, Donald Trump was pounced on by the media and opponents alike for this statement in March, and accused of dog-whistling a less than veiled threat.
“If you disenfranchise those people, and you say, ‘I’m sorry, you’re 100 votes short’…I think you’d have problems like you’ve never seen before. I think bad things would happen.... (when pushed, he qualified it by saying) .... I think you'd have riots"
It's really not too far removed from
"and if people feel that voting doesn't change anything, then violence is the next step"
Both candidates are talking about a situation where they fail to get the result they want, and suggetsing that this would result in violence. The American interpretation of Trump's dog-whistle is very different though.
By contrast, you would suggest he is a prophet making a forecast of what would happen. They basically called him out on it as making a threat. It was widely held by commentators and analysts that the dog whistle in this case was being aimed at his own supporters (legitimacy) and the executive branch of the GOP (the target)
Incidentally, Donald Trump is equally adept at utilising the other qualifer that you suggest gives him a pass out too.
"Some people call her Pocahontas" - Trump in the last couple of weeks trying to mock Sen Elizabeth Warren over her native Cherokee ancestry - (which is a bit questionable in truth).
There's only one person who calls Elizabeth Warren, Pocahontas (or did do) and that's Trump. I haven't checked the tweets that his echo chambers produce, but would be preapred to guess that there is a strong likelihood others have followed him now.
Mind you, other commentators also suggest that Trump has done away with the traditional dog-whistle to appeal to the receptive race voter, and gone "full fog horn"
For context, it's also worth acknowledging that Trump has also called directly for violence against protestors, even offering to pay the legal bills of anyone who takes up his challenge. No one is accusing Farage of having stepped over this threshold